Overview.bs

<pre class='metadata'>
Title: Selectors Level 4
Group: CSSWG
Shortname: selectors
Level: 4
Status: ED
Work Status: Refining
ED: https://un5n798jx6qx6j0rmf2verhh.julianrbryant.com/selectors/
TR: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/selectors-4/
Previous Version: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2022/WD-selectors-4-20220507/
Previous Version: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2018/WD-selectors-4-20181121/
Previous Version: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2018/WD-selectors-4-20180202/
Previous Version: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2018/WD-selectors-4-20180201/
Previous Version: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2013/WD-selectors4-20130502/
Previous Version: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2012/WD-selectors4-20120823/
Previous Version: https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2011/WD-selectors4-20110929/
Editor: Elika J. Etemad / fantasai, Apple, https://umn6effpw2gvaenh3jatp4m31zg12ar.julianrbryant.com/contact, w3cid 35400
Editor: Tab Atkins Jr., Google, https://umnb2f9fwamm0.julianrbryant.com/contact/, w3cid 42199
Former Editor: Tantek Çelik, https://umn0mtkzghpyunj0h41g.julianrbryant.com
Former Editor: Daniel Glazman
Former Editor: Ian Hickson
Former Editor: Peter Linss
Former Editor: John Williams
Abstract: <a>Selectors</a> are patterns that match against elements in a tree, and as such form one of several technologies that can be used to select nodes in a document. Selectors have been optimized for use with HTML and XML, and are designed to be usable in performance-critical code. They are a core component of <abbr title="Cascading Style Sheets">CSS</abbr> (Cascading Style Sheets), which uses Selectors to bind style properties to elements in the document.
Abstract: Selectors Level 4 describes the selectors that already exist in [[!SELECT]], and further introduces new selectors for CSS and other languages that may need them.
At Risk: the column combinator
At Risk: [=user action pseudo-classes=] applying to non-[=tree-abiding pseudo-elements=]
Ignored Terms: function token, Document, DocumentFragment, math, h1, shadow tree, querySelector(), quirks mode, button, a, span, object, p, div, q, area, link, label, input, html, em, li, ol, pre, CSS Value Definition Syntax
Ignored Vars: identifier, i
Ignore MDN Failure: #blank-pseudo
Ignore MDN Failure: #the-column-combinator
Ignore MDN Failure: #the-future-pseudo
Ignore MDN Failure: #the-past-pseudo
Ignore MDN Failure: #the-target-within-pseudo
WPT Path Prefix: css/selectors/
WPT Display: closed
</pre>
<pre class=link-defaults>
spec:css-values-4; type:dfn; text:identifier
spec:css-display-3; type:property; text:display
spec:css-display-4; type:property; text:visibility
spec:css-pseudo-4; type:selector;
	text: ::before
	text: ::after
	text: ::first-line
	text: ::first-letter
spec:dom; type:dfn; text:descendant
spec:html; type:dfn; text:muted
spec:html; type:element-attr; for:a; text:href
spec:html; type:element;
	text:meta
	text:audio
spec:selectors-5; type:selector; text::blank
</pre>

<style>
#selector-examples td:first-child {
	white-space: nowrap;
}
</style>

<h2 id="context">
Introduction</h2>

	<em>This section is not normative.</em>

	<wpt title="This section is not normative, it does not need tests."></wpt>

	A <a>selector</a> is a boolean predicate
	that takes an element in a tree structure
	and tests whether the element matches the selector or not.

	These expressions may be used for many things:

	<ul>
		<li>
			directly on an element to test whether it matches some criteria,
			such as in the <code>element.matches()</code> function defined in [[DOM]]

		<li>
			applied to an entire tree of elements
			to filter it into a set of elements that match the criteria,
			such as in the <code>document.querySelectorAll()</code> function defined in [[DOM]]
			or the selector of a CSS style rule.

		<li>
			used "in reverse" to generate markup that would match a given selector,
			such as in <a href="https://umn2j2h8gjn62.julianrbryant.com/">HAML</a> or <a href="https://un5qgjbzw9dxcq3ecfxberhh.julianrbryant.com/wiki/Emmet_(software)">Emmet</a>.
	</ul>

	Selectors Levels 1, 2, and 3 are defined as the subsets of selector
	functionality defined in the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/REC-CSS1">CSS1</a>,
	<a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/CSS21/">CSS2.1</a>, and
	<a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/css3-selectors/">Selectors Level 3</a>
	specifications, respectively. This module defines Selectors Level 4.

<h3 id="placement">Module Interactions</h3>

	<wpt title="Tests not needed for this section."></wpt>

	This module replaces the definitions of
	and extends the set of selectors defined for CSS in [[SELECT]] and [[CSS21]].

	Pseudo-element selectors,
	which define abstract elements in a rendering tree,
	are not part of this specification:
	their generic syntax is described here,
	but, due to their close integration with the rendering model and irrelevance to other uses such as DOM queries,
	they will be defined in other modules.

<h2 id="overview">
Selectors Overview</h2>

	<em>This section is non-normative, as it merely summarizes the
	following sections.</em>

	<wpt title="This section is not normative, it does not need tests."></wpt>

	A selector represents a structure. This structure can be used as a
	condition (e.g. in a CSS rule) that determines which elements a
	selector matches in the document tree, or as a flat description of the
	HTML or XML fragment corresponding to that structure.

	Selectors may range from simple element names to rich contextual
	representations.

	The following table summarizes the Selector syntax:

	<table class="data" id="selector-examples">
		<col class="pattern">
		<col class="meaning">
		<col class="section">
		<col class="level">
		<thead>
			<tr>
				<th>Pattern
				<th>Represents
				<th>Section
				<th>Level
		<tbody>
			<tr>
				<td><code>*</code>
				<td>any element
				<td>[[#the-universal-selector]]
				<td>2
			<tr>
				<td><code>E</code>
				<td>an element of type E
				<td>[[#type-selectors]]
				<td>1
		<tbody>
			<tr>
				<td><code>E:not(<var>s1</var>, <var>s2</var>, &hellip;)</code>
				<td>an E element that does not match either <a>compound selector</a> <var>s1</var>
						or <a>compound selector</a> <var>s2</var>
				<td>[[#negation]]
				<td>3/4
			<tr>
				<td><code>E:is(<var>s1</var>, <var>s2</var>, &hellip;)</code>
				<td>an E element that matches <a>compound selector</a> <var>s1</var>
						and/or <a>compound selector</a> <var>s2</var>
				<td>[[#matches]]
				<td>4
			<tr>
				<td><code>E:where(<var>s1</var>, <var>s2</var>, &hellip;)</code>
				<td>an E element that matches <a>compound selector</a> <var>s1</var>
						and/or <a>compound selector</a> <var>s2</var> but contributes no specificity.
				<td>[[#zero-matches]]
				<td>4
			<tr>
				<td><code>E:has(<var>rs1</var>, <var>rs2</var>, &hellip;)</code>
				<td>an E element,
					if there exists an element that matches
					either of the <a>relative selectors</a> <var>rs1</var> or <var>rs2</var>,
					when evaluated with E as the <a>anchor elements</a>
				<td>[[#relational]]
				<td>4
		<tbody>
			<tr>
				<td><code>E.warning</code>
				<td>an E element belonging to the class <code>warning</code>
						(the document language specifies how class is determined).
				<td>[[#class-html]]
				<td>1
			<tr>
				<td><code>E#myid</code>
				<td>an E element with ID equal to <code>myid</code>.
				<td>[[#id-selectors]]
				<td>1
			<tr>
				<td><code>E[foo]</code>
				<td>an E element with a <code>foo</code> attribute
				<td>[[#attribute-representation]]
				<td>2
			<tr>
				<td><code>E[foo="bar"]</code>
				<td>an E element whose <code>foo</code> attribute value is
						exactly equal to <code>bar</code>
				<td>[[#attribute-representation]]
				<td>2
			<tr>
				<td><code>E[foo="bar" i]</code>
				<td>an E element whose <code>foo</code> attribute value is
						exactly equal to any (ASCII-range) case-permutation of <code>bar</code>
				<td>[[#attribute-case]]
				<td>4
			<tr>
				<td><code>E[foo="bar" s]</code>
				<td>an E element whose <code>foo</code> attribute value is
						[=identical to=] <code>bar</code>
				<td>[[#attribute-case]]
				<td>4
			<tr>
				<td><code>E[foo~="bar"]</code>
				<td>an E element whose <code>foo</code> attribute value is
						a list of whitespace-separated values, one of which is
						exactly equal to <code>bar</code>
				<td>[[#attribute-representation]]
				<td>2
			<tr>
				<td><code>E[foo^="bar"]</code>
				<td>an E element whose <code>foo</code> attribute value
						begins exactly with the string <code>bar</code>
				<td>[[#attribute-substrings]]
				<td>3
			<tr>
				<td><code>E[foo$="bar"]</code>
				<td>an E element whose <code>foo</code> attribute value
						ends exactly with the string <code>bar</code>
				<td>[[#attribute-substrings]]
				<td>3
			<tr>
				<td><code>E[foo*="bar"]</code>
				<td>an E element whose <code>foo</code> attribute value
						contains the substring <code>bar</code>
				<td>[[#attribute-substrings]]
				<td>3
			<tr>
				<td><code>E[foo|="en"]</code>
				<td>an E element whose <code>foo</code> attribute value is
						a hyphen-separated list of values beginning with <code>en</code>
				<td>[[#attribute-representation]]
				<td>2
		<tbody>
			<tr>
				<td><code>E:dir(ltr)</code>
				<td>an element of type E with left-to-right directionality
					(the document language specifies how directionality is determined)
				<td>[[#the-dir-pseudo]]
				<td>4
			<tr>
				<td><code>E:lang(zh, "*-hant")</code>
				<td>an element of type E tagged as being either in Chinese
					(any dialect or writing system)
					or otherwise written with traditional Chinese characters
				<td>[[#the-lang-pseudo]]
				<td>2/4
		<tbody>
			<tr>
				<td><code>E:any-link</code>
				<td>an E element being the source anchor of a hyperlink
				<td>[[#the-any-link-pseudo]]
				<td>4
			<tr>
				<td><code>E:link</code>
				<td>an E element being the source anchor of a hyperlink
						of which the target is not yet visited
				<td>[[#link]]
				<td>1
			<tr>
				<td><code>E:visited</code>
				<td>an E element being the source anchor of a hyperlink
						of which the target is already visited
				<td>[[#link]]
				<td>1
			<tr>
				<td><code>E:target</code>
				<td>an E element being the target of the current URL
				<td>[[#the-target-pseudo]]
				<td>3
			<tr>
				<td><code>E:scope</code>
				<td>an E element being a [=scoping root=]
				<td>[[#the-scope-pseudo]]
				<td>4
		<tbody>
			<tr>
				<td><code>E:active</code>
				<td>an E element that is in an activated state
				<td>[[#the-active-pseudo]]
				<td>1
			<tr>
				<td><code>E:hover</code>
				<td>an E element that is under the cursor,
					or that has a descendant under the cursor
				<td>[[#the-hover-pseudo]]
				<td>2
			<tr>
				<td><code>E:focus</code>
				<td>an E element that has user input focus
				<td>[[#the-focus-pseudo]]
				<td>2
			<tr>
				<td><code>E:focus-within</code>
				<td>an E element that has user input focus or contains an element that has input focus.
				<td>[[#the-focus-within-pseudo]]
				<td>4
			<tr>
				<td><code>E:focus-visible</code>
				<td>an E element that has user input focus,
					and the UA has determined that a focus ring or other indicator
					should be drawn for that element
				<td>[[#the-focus-visible-pseudo]]
				<td>4
		<tbody>
			<tr>
				<td><code>E:enabled<br>E:disabled</code>
				<td>a user interface element E that is enabled or disabled, respectively
				<td>[[#enableddisabled]]
				<td>3
			<tr>
				<td><code>E:read-write</code><br><code>E:read-only</code>
				<td>a user interface element E that is user alterable, or not
				<td>[[#rw-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:placeholder-shown</code>
				<td>an input control currently showing placeholder text
				<td>[[#placeholder]]
				<td>3-UI/4
			<tr>
				<td><code>E:default</code>
				<td>a user interface element E that is the default item in a group of related choices
				<td>[[#the-default-pseudo]]
				<td>3-UI/4
			<tr>
				<td><code>E:checked</code><br><code>E:unchecked</code><br><code>E:indeterminate</code>
				<td>a user interface element E that is checked/selected
					(for instance a radio-button or checkbox),
					unchecked,
					or in an indeterminate state
					(neither checked nor unchecked)
				<td>[[#input-value-states]]
				<td>3
			<tr>
				<td><code>E:valid</code><br><code>E:invalid</code>
				<td>a user-input element E that meets, or doesn't, its data validity semantics
				<td>[[#validity-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:in-range</code><br><code>E:out-of-range</code>
				<td>a user-input element E whose value is in-range/out-of-range
				<td>[[#range-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:required</code><br><code>E:optional</code>
				<td>a user-input element E that requires/does not require input
				<td>[[#opt-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:user-invalid</code>
				<td>a user-altered user-input element E with incorrect input (invalid, out-of-range, omitted-but-required)
				<td>[[#user-pseudos]]
				<td>4
		<tbody>
			<tr>
				<td><code>E:root</code>
				<td>an E element, root of the document
				<td>[[#the-root-pseudo]]
				<td>3
			<tr>
				<td><code>E:empty</code>
				<td>an E element that has no children (neither elements nor text) except perhaps white space
				<td>[[#the-empty-pseudo]]
				<td>3
			<tr>
				<td><code>E:nth-child(<var>n</var> [of <var>S</var>]?)</code>
				<td>an E element, the <var>n</var>-th child of its parent matching <var>S</var>
				<td>[[#the-nth-child-pseudo]]
				<td>3/4
			<tr>
				<td><code>E:nth-last-child(<var>n</var> [of <var>S</var>]?)</code>
				<td>an E element, the <var>n</var>-th child of its parent matching <var>S</var>,
						counting from the last one
				<td>[[#the-nth-last-child-pseudo]]
				<td>3/4
			<tr>
				<td><code>E:first-child</code>
				<td>an E element, first child of its parent
				<td>[[#the-first-child-pseudo]]
				<td>2
			<tr>
				<td><code>E:last-child</code>
				<td>an E element, last child of its parent
				<td>[[#the-last-child-pseudo]]
				<td>3
			<tr>
				<td><code>E:only-child</code>
				<td>an E element, only child of its parent
				<td>[[#the-only-child-pseudo]]
				<td>3
			<tr>
				<td><code>E:nth-of-type(<var>n</var>)</code>
				<td>an E element, the <var>n</var>-th sibling of its type
				<td>[[#the-nth-of-type-pseudo]]
				<td>3
			<tr>
				<td><code>E:nth-last-of-type(<var>n</var>)</code>
				<td>an E element, the <var>n</var>-th sibling of its type,
						counting from the last one
				<td>[[#the-nth-last-of-type-pseudo]]
				<td>3
			<tr>
				<td><code>E:first-of-type</code>
				<td>an E element, first sibling of its type
				<td>[[#the-first-of-type-pseudo]]
				<td>3
			<tr>
				<td><code>E:last-of-type</code>
				<td>an E element, last sibling of its type
				<td>[[#the-last-of-type-pseudo]]
				<td>3
			<tr>
				<td><code>E:only-of-type</code>
				<td>an E element, only sibling of its type
				<td>[[#the-only-of-type-pseudo]]
				<td>3
		<tbody>
			<tr>
				<td><code>E F</code>
				<td>an F element descendant of an E element
				<td>[[#descendant-combinators]]
				<td>1
			<tr>
				<td><code>E > F</code>
				<td>an F element child of an E element
				<td>[[#child-combinators]]
				<td>2
			<tr>
				<td><code>E + F</code>
				<td>an F element immediately preceded by an E element
				<td>[[#adjacent-sibling-combinators]]
				<td>2
			<tr>
				<td><code>E ~ F</code>
				<td>an F element preceded by an E element
				<td>[[#general-sibling-combinators]]
				<td>3
	</table>

	Note: Some Level 4 selectors (noted above as "3-UI") were introduced in [[CSS3UI]].

	<wpt>
		old-tests/css3-modsel-1.xml
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		old-tests/css3-modsel-147b.xml
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		old-tests/css3-modsel-87.xml
		old-tests/css3-modsel-87b.xml
		old-tests/css3-modsel-88.xml
		old-tests/css3-modsel-88b.xml
		old-tests/css3-modsel-89.xml
		old-tests/css3-modsel-9.xml
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		old-tests/css3-modsel-90b.xml
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		old-tests/css3-modsel-92.xml
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		old-tests/css3-modsel-94b.xml
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		old-tests/css3-modsel-96.xml
		old-tests/css3-modsel-96b.xml
		old-tests/css3-modsel-97.xml
		old-tests/css3-modsel-97b.xml
		old-tests/css3-modsel-98.xml
		old-tests/css3-modsel-98b.xml
		old-tests/css3-modsel-99.xml
		old-tests/css3-modsel-99b.xml
		old-tests/css3-modsel-d1.xml
		old-tests/css3-modsel-d1b.xml
		old-tests/css3-modsel-d2.xml
		old-tests/css3-modsel-d3.xml
		old-tests/css3-modsel-d4.xml
	</wpt>

	<wpt title="Tests that do not relate to any section">
		eof-right-after-selector-crash.html
		eof-some-after-selector-crash.html
		hash-collision.html
		parsing/invalid-pseudos.html
		selector-after-font-family.html
	</wpt>

<h2 id="syntax">
Selector Syntax and Structure</h2>

<h3 id="structure">
Structure and Terminology</h3>

	A <dfn export>selector</dfn> represents
	a particular pattern of element(s) in a tree structure.
	The term <a>selector</a> can refer to a <a>simple selector</a>,
	<a>compound selector</a>, <a>complex selector</a>, or <a>selector list</a>.
	The <dfn export for=selector lt="subject|subject of a selector|subject of the selector">subject of a selector</dfn> is
	any element that selector is defined to be about;
	that is, any element <dfn export lt="match">matching</dfn> that <a>selector</a>.

	A <dfn id="simple" export>simple selector</dfn>
	is a single condition on an element.
	A <a>type selector</a>,
	<a>universal selector</a>,
	<a>attribute selector</a>,
	<a>class selector</a>,
	<a>ID selector</a>,
	or <a>pseudo-class</a>
	is a <a>simple selector</a>.
	(It is represented by <<simple-selector>>
	in the selectors <a href="#grammar">grammar</a>.)
	A given element is said to <a>match</a> a <a>simple selector</a>
	when that <a>simple selector</a>,
	as defined in this specification and in accordance with the <a>document language</a>,
	accurately describes the element.

	A <dfn id="compound" export>compound selector</dfn>
	is a sequence of <a>simple selectors</a>
	that are not separated by a <a for=selector>combinator</a>,
	and represents a set of simultaneous conditions on a single element.
	If it contains a <a>type selector</a> or <a>universal selector</a>,
	that selector must come first in the sequence.
	Only one type selector or universal selector is allowed in the sequence.
	(A <a>compound selector</a> is represented by <<compound-selector>>
	in the selectors <a href="#grammar">grammar</a>.)
	A given element is said to <a>match</a> a <a>compound selector</a>
	when it matches all <a>simple selectors</a> in the <a>compound selector</a>.

	Note: As whitespace represents the <a>descendant combinator</a>,
	no whitespace is allowed between the <a>simple selectors</a>
	in a <a>compound selector</a>.

	A <dfn id=pseudo-compound>pseudo-compound selector</dfn>
	is a [=pseudo-element=] selector,
	optionally followed by additional [=pseudo-class=] selectors,
	and optionally preceded by a [=compound selector=]
	or another [=pseudo-compound selector=],
	without any [=selector/combinators=].
	(A <a>pseudo-compound selector</a> is represented by <<pseudo-compound-selector>>
	in the selectors <a href="#grammar">grammar</a>.)
	A [=pseudo-element=] [=matches=] a [=pseudo-compound selector=]
	when it has the specified pseudo-element name,
	matches the additional conditions represented by any [=pseudo-classes=],
	and has an [=originating element=]
	represented by the adjacent preceding selector.
	If there is no adjacent preceding selector,
	the [=universal selector=] is assumed.
	(For example, ''.foo ::before''
	is equivalent to ''.foo *::before'',
	and distinct from ''.foo::before''.)

	<div class=example>
		For example, in ''.foo::before:hover'',
		the ''.foo'' is a [=compound selector=],
		while the ''::before:hover'' is a [=pseudo-compound selector=].
		However, in ''.foo::before::marker'',
		''::before'' and ''::marker'' are separate [=pseudo-compound selectors=].
	</div>

	Note: A [=pseudo-compound selector=] <strong>is not</strong>
	a [=compound selector=],
	and can't be used in places that expect a [=compound selector=] only.
	[=Pseudo-compound selectors=] act as if they carry a [=selector/combinator=] with themselves,
	expressing their relationship with their [=originating element=],
	just as the ''>'' combinator
	expresses a relationship with a parent element.

	A <dfn export for=selector>combinator</dfn>
	is a condition of relationship between two elements
	represented by the <a>compound selectors</a> on either side.
	Combinators in Selectors Level 4 include:
	the <a>descendant combinator</a> (white space),
	the <a>child combinator</a> (U+003E, <code>></code>),
	the <a>next-sibling combinator</a> (U+002B, <code>+</code>),
	and the <a>subsequent-sibling combinator</a> (U+007E, <code>~</code>).
	Two given elements are said to <a>match</a> a <a for=selector>combinator</a>
	when the condition of relationship between these elements is true.

	A <dfn id="complex" export>complex selector</dfn> is
	a sequence of one or more <a>compound selectors</a>
	and/or [=pseudo-compound selectors=],
	with [=compound selectors=] separated by <a for=selector>combinators</a>.
	It represents a set of simultaneous conditions
	on a set of elements in the particular relationships
	described by its <a for=selector>combinators</a>.
	(Complex selectors are represented by <<complex-selector>>
	in the selectors <a href="#grammar">grammar</a>.)
	A given element or pseudo-element
	is said to <a>match</a> a <a>complex selector</a>
	when it matches the final [=compound selector|compound=]/[=pseudo-compound selector=] in the sequence,
	and every preceding unit of the sequence also [=matches=]
	an element or [=pseudo-element=],
	with the correct relationship between consecutive units
	as expressed by the combinators separating them
	(or, for [=pseudo-compound selectors=],
	the correct [=originating element=] relationship).

	<div class=example>
		For example, ''.foo.bar'' matches an element
		with both "foo" and "bar" classes.

		''.ancestor > .foo.bar'' matches a subset of those elements:
		only those whose parent element
		(as indicated by the ''>'' combinator)
		has the "ancestor" class.

		''.foo.bar::before'' matches a ''::before'' pseudo-element,
		whose [=originating element=] matches ''.foo.bar''.
	</div>

	A <dfn export lt="list of simple selectors|list of compound selectors|list of complex selectors">list of simple/compound/complex selectors</dfn>
	is a comma-separated list of
	<a lt="simple selector">simple</a>,
	<a lt="compound selector">compound</a>,
	or <a>complex selectors</a>.
	This is also called just a <dfn export lt="selector list|list of selectors">selector list</dfn>
	when the type is either unimportant or specified in the surrounding prose;
	if the type is important and unspecified,
	it defaults to meaning a <a>list of complex selectors</a>.
	(See [[#grouping]] for additional information on <a>selector lists</a>
	and the various &lt;*-selector-list> productions in the <a href="#grammar">grammar</a>
	for their formal syntax.)
	A given element is said to <a>match</a> a <a>selector list</a>
	when it matches any (at least one) of the <a>selectors</a>
	in that <a>selector list</a>.

<h3 id='data-model'>
Data Model</h3>

	Selectors are evaluated against an element tree such as the DOM. [[!DOM]]
	Within this specification,
	this may be referred to as the "document tree" or "source document".

	Each element may have any of the following five aspects,
	which can be selected against,
	all of which are matched as strings:

	<ul>
		<li>The element's type (also known as its tag name).
		<li>The element's namespace.
		<li>An ID.
		<li>Classes (named groups) to which it belongs.
		<li>Attributes, which are name-value pairs.
	</ul>

	Many of the selectors depend on the semantics of the <dfn export>document language</dfn>
	(i.e. the language and semantics of the document tree)
	and/or the semantics of the <dfn>host language</dfn>
	(i.e. the language that is using selectors syntax).
	For example, the '':lang()'' selector depends on the <a>document language</a> (e.g. HTML)
	to define how an element is associated with a language.
	As a slightly different example, the ''::first-line'' pseudo-element
	depends on the <a>host language</a> (e.g. CSS)
	to define what a ''::first-line'' pseudo-element represents
	and what it can do.

<h4 id=featureless-elements>
Featureless Elements</h4>

	<wpt>
		featureless-001.html
		featureless-002.html
		featureless-003.html
		featureless-004.html
		featureless-005.html
	</wpt>

	While individual elements may lack any of the above features,
	some elements are <dfn export>featureless</dfn>.
	A <a>featureless</a> element does not match <em>any selector at all</em>,
	except:

	* [=simple selectors=] it is explicitly defined to match
	* [=compound selectors=],
		if all contained [=simple selectors=]
		are allowed to match it
	* [=complex selectors=],
		if the [=compound selector=] targeting the [=selector/subject=]
		is allowed to match it
	* [=selector lists=],
		if at least one selector in the list
		is allowed to match it
	* [=logical combination pseudo-classes=],
		if their argument selector is allowed to match it
	* the '':has()'' pseudo-class,
		if and only if the [=compound selector=] it's part of
		contains at least one <em>other</em> [=simple selector=]
		that's allowed to match it.

	If a selector would otherwise match a [=featureless=] element,
	except for the existence of the default namespace [[CSS-NAMESPACES-3]]
	(because [=featureless=] elements do not have a namespace
	unless otherwise defined),
	the default namespace does not prevent the match.

	<div class='example'>
		For example, the [=shadow host=] in a [=shadow tree=] is [=featureless=],
		and can't be matched by <em>any</em> [=pseudo-class=]
		except for '':host'' and '':host-context()''
		(or combinations including those,
		such as '':is(:host, :root)'').

		Logical combinations like '':not(.foo:host)''
		will never match the host element
		(even if it doesn't have a "foo" class),
		because not all of the simple selectors in ''.foo:host''
		are allowed to match the [=shadow host=].

		Similarly, '':not(:host > .foo)'' will never match the [=shadow host=],
		even tho the [=shadow host=] is indeed *not* a descendant of itself
		and doesn't have the "foo" class,
		because the subject of the complex selector argument (''.foo'')
		isn't allowed to match the [=shadow host=].
	</div>

	<div class='example'>
		In general, you can't match a [=featureless=] element
		without explicitly using one of the [=simple selectors=]
		it's allowed to match,
		to avoid accidentally selecting one of these elements
		(which are otherwise <em>intentionally</em> easy to not think about).
		For example, ''*'' will never match a [=featureless=] element.

		The rule for '':has()'', above, works similarly.
		Even if a [=shadow host=] contains a ''.foo'' descendant,
		'':has(.foo)'' will not match it,
		because <em>the rest</em> of the compound selector
		(empty)
		doesn't contain a simple selector that can match the host.
		You have to write '':host:has(.foo)'' in order to match the host element.
	</div>


<h3 id="scoping">
Scoped Selectors</h3>

	Some host applications may choose to <dfn export lt="scoped selector" local-lt="scope">scope</dfn> selectors
	to a particular subtree or fragment of the document,
	The root of the scoping subtree is called the <dfn export>scoping root</dfn>.

	When a selector is <a>scoped</a>,
	it matches an element only if the element is a descendant of the <a>scoping root</a>.
	(The rest of the selector can match unrestricted;
	it's only the final matched elements that must be within the scope.)

	<div class='example'>
		For example,
		the {{Element/querySelector()}} method defined in [[DOM]]
		allows the author to evaluate a <a>scoped</a> selector
		relative to the element it's called on.

		A call like <code highlight=js>widget.querySelector("a")</code>
		will thus only find <{a}> elements inside of the <code>widget</code> element,
		ignoring any other <{a}>s that might be scattered throughout the document.
	</div>


<h3 id="relative">
Relative Selectors</h3>

	Certain contexts may accept <dfn lt="relative selector" local-lt="relative" export>relative selectors</dfn>,
	which are a shorthand for selectors that represent elements relative to one or more
	<dfn export local-lt="anchor element">relative selector anchor elements</dfn>.
	Relative selectors begin with a <a for=selector>combinator</a>,
	with a selector representing the [=anchor element=]
	implied at the start of the selector.
	(If no combinator is present,
	the [=descendant combinator=] is implied.)

	Relative selectors are represented by <<relative-selector>> in the selectors <a href="#grammar">grammar</a>,
	and lists of them by <<relative-selector-list>>.


<h3 id="pseudo-classes">
Pseudo-classes</h3>

	<dfn export id="pseudo-class" lt="pseudo-class">Pseudo-classes</dfn> are <a>simple selectors</a>
	that permit selection based on
	information that lies outside of the document tree
	or that can be awkward or impossible to express using the other simple selectors.
	They can also be dynamic,
	in the sense that an element can acquire or lose a pseudo-class
	while a user interacts with the document,
	without the document itself changing.
	<a>Pseudo-classes</a> do not appear in or modify the document source or document tree.

	The syntax of a <a>pseudo-class</a>
	consists of a ":" (U+003A COLON)
	followed by the name of the <a>pseudo-class</a>
	as a CSS <a>identifier</a>,
	and, in the case of a <dfn export id="functional-pseudo-class">functional pseudo-class</dfn>,
	a pair of parentheses containing its arguments.

	<p class="example">
		For example, '':valid'' is a regular pseudo-class,
		and  '':lang()'' is a <a>functional pseudo-class</a>.

	Like all CSS keywords, <a>pseudo-class</a> names are <a>ASCII case-insensitive</a>.
	No <a>white space</a> is allowed between the colon and the name of the <a>pseudo-class</a>,
	nor, as usual for CSS syntax,
	between a <a>functional pseudo-class</a>’s name and its opening parenthesis
	(which thus form a CSS <a>function token</a>).
	Also as usual,
	<a>white space</a> is allowed around the arguments inside the parentheses
	of a functional pseudo-class
	unless otherwise specified.

	Like other <a>simple selectors</a>,
	<a>pseudo-classes</a> are allowed in all <a>compound selectors</a> contained in a selector,
	and must follow the <a>type selector</a> or <a>universal selector</a>, if present.

	Note: Some <a>pseudo-classes</a> are mutually exclusive
	(such that a <a>compound selector</a> containing them, while valid, will never match anything),
	while others can apply simultaneously to the same element.


<h3 id="pseudo-elements">Pseudo-elements</h3>

	<wpt>
		x-pseudo-element.html
	</wpt>

	Similar to how certain <a>pseudo-classes</a> represent additional state information
	not directly present in the document tree,
	a <dfn export id="pseudo-element" lt="pseudo-elements">pseudo-element</dfn> represents an <em>element</em>
	not directly present in the document tree.
	They are used to create abstractions about the document tree
	beyond those provided by the document tree.
	For example,
	pseudo-elements can be used to select portions of the document
	that do not correspond to a document-language element
	(including such ranges as don't align to element boundaries or fit within its tree structure);
	that represent content not in the document tree or in an alternate projection of the document tree;
	or that rely on information provided by styling, layout, user interaction, and other processes that are not reflected in the document tree.

	<div class="example">
		For instance, document languages do not offer mechanisms to access
		the first letter or first line of an element's content,
		but there exist <a>pseudo-elements</a>
		(''::first-letter'' and ''::first-line'')
		that allow those things to be styled.
		Notice especially that in the case of ''::first-line'',
		which portion of content is represented by the pseudo-element
		depends on layout information
		that cannot be inferred from the document tree.

		<a>Pseudo-elements</a> can also represent content that doesn't exist in the source document at all,
		such as the ''::before'' and ''::after'' pseudo-elements
		which allow additional content to be inserted before or after the contents of any element.
	</div>

	Like <a>pseudo-classes</a>,
	<a>pseudo-elements</a> do not appear in or modify the document source or document tree.
	Accordingly, they also do not affect the interpretation of <a>structural pseudo-classes</a>
	or other selectors pertaining to their <a>originating element</a> or its tree.

	The host language defines which pseudo-elements exist, their type, and their abilities.
	Pseudo-elements that exist in CSS
	are defined in [[CSS21]] (Level 2), [[SELECT]] (Level 3), and [[CSS-PSEUDO-4]] (Level 4).

<h4 id="pseudo-element-syntax">
Syntax</h4>

	The syntax of a <a>pseudo-element</a>
	is "::" (two U+003A COLON characters)
	followed by the name of the <a>pseudo-element</a> as an <a>identifier</a>,
	and, in the case of a <dfn export id="functional-pseudo-element">functional pseudo-element</dfn>,
	a pair of parentheses containing its arguments.
	<a>Pseudo-element</a> names are <a>ASCII case-insensitive</a>.
	No <a>white space</a> is allowed between the two colons, or between the colons and the name.

	Because <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/CSS1">CSS Level 1</a> and <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/CSS2">CSS Level 2</a>
	conflated pseudo-elements and pseudo-classes by sharing a single-colon syntax for both,
	user agents must also accept the previous one-colon notation
	for the Level 1 &amp; 2 pseudo-elements
	(''::before'', ''::after'', ''::first-line'', and ''::first-letter'').
	This compatibility notation is not allowed for any other <a>pseudo-elements</a>.
	However, as this syntax is deprecated,
	authors should use the Level 3+ double-colon syntax for these <a>pseudo-elements</a>.

	<a>Pseudo-elements</a> are <a>featureless</a>,
	and so can't be matched by any other selector.

<h4 id="pseudo-element-attachment">
Binding to the Document Tree</h4>

	<a>Pseudo-elements</a> do not exist independently in the tree:
	they are always bound to another element on the page,
	called their <dfn export>originating element</dfn>.
	Syntactically, a <a>pseudo-element</a> immediately follows
	the <a>compound selector</a> representing its <a>originating element</a>.
	If this <a>compound selector</a> is omitted,
	it is assumed to be the <a>universal selector</a> ''*''.

	<div class='example'>
		For example, in the selector ''div a::before'',
		the ''a'' elements matched by the selector are the <a>originating elements</a>
		for the ''::before'' pseudo-elements attached to them.

		The selector ''::first-line'' is equivalent to ''*::first-line'',
		which selects the ''::first-line'' pseudo-element on <em>every</em> element in the document.
	</div>

	When a <a>pseudo-element</a> is encountered in a selector,
	the part of the selector before the <a>pseudo-element</a> selects the <a>originating element</a> for the <a>pseudo-element</a>;
	the part of the selector after it, if any, applies to the <a>pseudo-element</a> itself.
	(See below.)

<h4 id="pseudo-element-states">
Pseudo-classing Pseudo-elements</h4>

	Certain [=pseudo-elements=] may be immediately followed
	by any combination of certain [=pseudo-classes=],
	in which case the <a>pseudo-element</a> is represented only when it is in the corresponding state.
	This specification allows any <a>pseudo-element</a> to be followed
	by any combination of the [=logical combination pseudo-classes=]
	and the [=user action pseudo-classes=].
	Other specifications may allow additional [=pseudo-classes=]
	to be attached to particular [=pseudo-elements=].
	Combinations that are not explicitly allowed are [=invalid selectors=].

	Note: The [=logical combination pseudo-classes=]
	pass any restrictions on validity of selectors at their position
	to their arguments.

	<div class="example">
		For example, since the '':hover'' pseudo-class specifies
		that it can apply to any pseudo-element,
		''::first-line:hover'' will match when the first line is hovered.
		However, since neither '':focus'' nor ''::first-line''
		define that '':focus'' can apply to ''::first-line'',
		the selector ''::first-line:focus'' will never match anything.

		Notice that ''::first-line:hover'' is very different from '':hover::first-line'',
		which matches the first line of any originating element that is hovered!
		For example, '':hover::first-line'' also matches the first line of a paragraph
		when the second line of the paragraph is hovered,
		whereas ''::first-line:hover'' only matches if the first line itself is hovered.
	</div>

<h4 id="sub-pseudo-elements">
Sub-pseudo-elements</h4>

	Some [=pseudo-elements=] are able to be the [=originating element=]
	of other [=pseudo-elements=],
	which are defined as the <dfn export lt="sub-pseudo-element">sub-pseudo-elements</dfn>
	of this <dfn export>originating pseudo-element</dfn>.
	For example,
	when ''::before'' is given a ''display/list-item'' [=display type=],
	it becomes the [=originating pseudo-element=]
	of its ''::before::marker'' [=sub-pseudo-element=].

	Where disambiguation is needed,
	the term <dfn export>ultimate originating element</dfn>
	refers to the real (non-pseudo) element
	from which a [=pseudo-element=] originates.

	Unless the corresponding [=sub-pseudo-element=]
	is explicitly defined to exist in another specification,
	pseudo-element selectors are not valid
	when compounded to another pseudo-element selector.
	So, for example, ''::before::before'' is an invalid selector,
	but ''::before::marker'' is valid
	(in implementations that support the ''::before::marker'' [=sub-pseudo-element=]).

<h4 id="pseudo-element-structure">
Internal Structure</h4>

	Some <a>pseudo-elements</a> are defined to have internal structure.
	These <a>pseudo-elements</a> may be followed by child/descendant combinators
	to express those relationships.
	Selectors containing <a for=selector>combinators</a> after the pseudo-element
	are otherwise invalid.

	<div class="example">
		For example, ''::first-letter + span'' and ''::first-letter em'' are invalid selectors.
		However, if a new ''::shadow'' pseudo-element were defined to have internal structure,
		''::shadow > p'' would be a valid selector.
	</div>

	Note: A future specification may expand the capabilities of existing pseudo-elements,
	so some of these currently-invalid selectors (e.g. ''::first-line :any-link'')
	may become valid in the future.

	The children of such <a>pseudo-elements</a> can simultaneously be children of other elements, too.
	However, at least in CSS, their rendering must be defined so as to maintain the tree-ness of the <a>box tree</a>.

	<!--
	<div class='example'>
		For example,
		the ''::slotted'' pseudo-element treats elements distributed to it as its children.
		This means that, given the following fragment:

		<pre>
		&lt;div>
			&lt;span>foo&lt;/span>
			&lt;"shadow root">
				&lt;content>&lt;/content>
			&lt;/"shadow root">
		&lt;/div>
		</pre>

		the selectors ''div > span'' and ''div::shadow ::slotted > span'' select the same element via different paths.

		However, when rendered,
		the <code>&lt;span></code> element generates boxes as if it were the child of the <code>&lt;content></code> element,
		rather than the <code>&lt;div></code> element,
		so the tree structure of the <a>box tree</a> is maintained.
	</div>
	-->

<h3 id="case-sensitive">
Characters and case sensitivity</h3>

	<wpt>
		case-insensitive-parent.html
		invalidation/quirks-mode-stylesheet-dynamic-add-001.html
		selectors-case-sensitive-001.html
	</wpt>

	All Selectors syntax is [=ASCII case-insensitive=]
	(i.e. [a-z]  and \[A-Z] are equivalent),
	except for the parts
	that are not under the control of Selectors:
	specifically,
	the case-sensitivity of
	document language element names,
	attribute names,
	and attribute values
	depends on the document language.

	<div class=example>
		For example,
		<a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selectors">in HTML, element and attribute names are ASCII case-insensitive</a>,
		but in XML, they are case-sensitive.
	</div>

	Case sensitivity of namespace prefixes is defined in [[!CSS3NAMESPACE]].
	Case sensitivity of <a>language ranges</a> is defined in the '':lang()'' section.

	<dfn id="whitespace">White space</dfn> in Selectors consists of the
	code points SPACE (U+0020), TAB (U+0009), LINE FEED (U+000A),
	CARRIAGE RETURN (U+000D), and FORM FEED (U+000C).
	Other space-like code points, such as EM SPACE (U+2003) and
	IDEOGRAPHIC SPACE (U+3000), are never considered syntactic white space.

	Code points in Selectors can be escaped with a backslash
	according to the same <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/CSS21/syndata.html#characters">escaping rules</a> as CSS. [[!CSS21]]
	Note that escaping a code point “cancels out”
	any special meaning it may have in Selectors.
	For example, the selector ''#foo>a'' contains a combinator,
	but ''#foo\>a'' instead selects an element with the id <code>foo>a</code>.


<h3 id="namespaces">
Declaring Namespace Prefixes</h3>

	Certain selectors support namespace prefixes.
	The mechanism by which namespace prefixes are <dfn id="nsdecl">declared</dfn>
	should be specified by the language that uses Selectors.
	If the language does not specify a namespace prefix declaration mechanism,
	then no prefixes are declared.
	In CSS, namespace prefixes are declared with the ''@namespace'' rule. [[!CSS3NAMESPACE]]

<h3 id="invalid">
Invalid Selectors and Error Handling</h3>

	<wpt>
		invalidation/selectorText-dynamic-001.html
		invalidation/sheet-going-away-001.html
		invalidation/sheet-going-away-002.html
	</wpt>

	User agents must observe the rules for handling
	<dfn export lt="invalid selector" local-lt="invalid">invalid selectors</dfn>:

	<ul>
		<li>a parsing error in a selector,
			e.g. an unrecognized token or a token which is not allowed at the current parsing point
			(see overall [[#grammar]] and per-selector syntax definitions),
			causes that selector to be invalid.
		<li>a simple selector containing an <a href="#namespaces">undeclared namespace prefix</a> is invalid
		<li>a selector containing an invalid simple selector, an invalid combinator
			or an invalid token is invalid.
		<li>a selector list containing an invalid selector is invalid.
		<li>an empty selector, i.e. one that contains no <a>compound selector</a>, is invalid.
	</ul>

	Note: Consistent with CSS’s forwards-compatible parsing principle,
	UAs <em>must</em> treat as <a>invalid</a>
	any pseudo-classes, pseudo-elements, combinators, or other syntactic constructs
	for which they have no usable level of support.
	See [[#w3c-partial]].

	An <a>invalid selector</a> represents, and therefore matches, nothing.


<h3 id="legacy-aliasing">
	Legacy Aliases</h3>

	Some selectors have a <dfn>legacy selector alias</dfn>.
	This is a name which, at parse time, is converted to the standard name
	(and thus does not appear anywhere in any object model representing the selector).


<h2 id="logical-combination">
Logical Combinations</h2>

	<wpt>
		invalidation/is-where-pseudo-containing-hard-pseudo-and-never-matching.html
		invalidation/is-where-pseudo-containing-hard-pseudo.html
	</wpt>

	Selector logic can be manipulated by
	[=compound selector|compounding=] (logical AND),
	[=selector lists=] (logical OR),
	and the <dfn>logical combination pseudo-classes</dfn>
	'':is()'', '':where()'', and '':not()''.
	The [=logical combination pseudo-classes=]
	are allowed anywhere that any other [=pseudo-classes=] are allowed,
	but pass any restrictions to their arguments.
	(For example, if only [=compound selectors=] are allowed,
	then only [=compound selectors=] are valid within an '':is()''.)

	Note: Since inside '':is()'' and '':where()''
	invalid arguments are dropped without invaliding the [=pseudo-class=] itself,
	selector arguments that are invalidated by contextual restrictions
	likewise do not invalidate the '':is()'' pseudo-class itself.


<h3 id="grouping">
Selector Lists</h3>

	A comma-separated list of selectors represents the union of all
	elements selected by each of the individual selectors in the
	<a>selector list</a>.
	(A comma is U+002C.) For example, in CSS when several selectors share
	the same declarations, they may be grouped into a comma-separated
	list. White space may appear before and/or after the comma.

	<div class="example">
		CSS example:
		In this example, we condense three rules with identical
		declarations into one. Thus,
		<pre>
		h1 { font-family: sans-serif }
		h2 { font-family: sans-serif }
		h3 { font-family: sans-serif }
		</pre>
		is equivalent to:
		<pre>h1, h2, h3 { font-family: sans-serif }
		</pre>
	</div>

	<strong>Warning</strong>: the equivalence is true in this example
	because all the selectors are valid selectors. If just one of these
	selectors were invalid, the entire <a>selector list</a> would be
	invalid. This would invalidate the rule for all three heading
	elements, whereas in the former case only one of the three individual
	heading rules would be invalidated.

	<div class="example">
		Invalid CSS example:

		<pre>
		h1 { font-family: sans-serif }
		h2..foo { font-family: sans-serif }
		h3 { font-family: sans-serif }
		</pre>

		is not equivalent to:

		<pre>h1, h2..foo, h3 { font-family: sans-serif } </pre>

		because the above selector (''h1, h2..foo, h3'')
		is entirely invalid and the entire style rule is dropped. (When
		the selectors are not grouped, only the rule for ''h2..foo''
		is dropped.)
	</div>


<h3 id="matches">
The Matches-Any Pseudo-class: '':is()''</h3>

	<wpt>
		invalidation/is.html
		is-default-ns-001.html
		is-default-ns-002.html
		is-default-ns-003.html
		is-default-ns-002.html
		is-default-ns-003.html
		is-nested.html
		is-specificity-shadow.html
		is-specificity.html
		is-where-basic.html
		is-where-error-crash.html
		is-where-error-recovery.html
		is-where-not.html
		is-where-pseudo-classes.html
		is-where-pseudo-elements.html
		is-where-shadow.html
		is-where-visited.html
		parsing/parse-is-where.html
		parsing/parse-is.html
		query/query-is.html
	</wpt>

	The matches-any pseudo-class, <dfn id='matches-pseudo'>:is()</dfn>,
	is a functional pseudo-class taking a <<forgiving-selector-list>> as its sole argument.

	If the argument, after parsing, is an empty list,
	the pseudo-class is valid but matches nothing.
	Otherwise, the pseudo-class matches any element
	that matches any of the selectors in the list.

	Note: The specificity of the '':is()'' pseudo-class
	is replaced by the specificity of its most specific argument.
	Thus, a selector written with '':is()''
	does not necessarily have equivalent specificity
	to the equivalent selector written without '':is()''
	For example, if we have
	'':is(ul, ol, .list) > [hidden]'' and ''ul > [hidden], ol > [hidden], .list > [hidden]''
	a ''[hidden]'' child of an <{ol}> matches the first selector
	with a specificity of (0,2,0)
	whereas it matches the second selector
	with a specificity of (0,1,1).
	See [[#specificity-rules]].

	Pseudo-elements cannot be represented by the matches-any pseudo-class;
	they are not valid within '':is()''.

	Default namespace declarations do not affect the <a>compound selector</a>
	representing the <a for=selector>subject</a> of any selector
	within a '':is()'' pseudo-class,
	unless that compound selector contains
	an explicit <a>universal selector</a> or <a>type selector</a>.

	<div class="example">
		For example, the following selector matches any element that is being
		hovered or focused, regardless of its namespace. In particular, it
		is not limited to only matching elements in the default namespace
		that are being hovered or focused.

		<pre>*|*:is(:hover, :focus) </pre>

		The following selector, however, represents only hovered or focused
		elements that are in the default namespace, because it uses an explicit
		universal selector within the '':is()'' notation:

		<pre>*|*:is(*:hover, *:focus) </pre>
	</div>

	As previous drafts of this specification
	used the name <dfn>:matches()</dfn> for this pseudo-class,
	UAs may additionally implement this obsolete name
	as a [=legacy selector alias=] for '':is()''
	if needed for backwards-compatibility.


<h3 id="negation">
The Negation (Matches-None) Pseudo-class: '':not()''</h3>

	<wpt>
		invalidation/not-001.html
		invalidation/not-002.html
		not-complex.html
		not-default-ns-001.html
		not-default-ns-002.html
		not-default-ns-003.html
		not-links.html
		not-specificity.html
		parsing/parse-not.html
		query/query-where.html
	</wpt>

	The negation pseudo-class, <dfn id='negation-pseudo'>:not()</dfn>,
	is a functional pseudo-class taking a <<complex-real-selector-list>> as an argument.
	It represents an element that is not represented by its argument.

	Note: In Selectors Level 3,
	only a single <a>simple selector</a> was allowed as the argument to '':not()''.

	Note: The specificity of the '':not()'' pseudo-class
	is replaced by the specificity of the most specific selector in its argument;
	thus it has the exact behavior of '':not(:is(<var ignore>argument</var>))''.
	See [[#specificity-rules]].

	Pseudo-elements cannot be represented by the negation pseudo-class;
	they are not valid within '':not()''.

	<div class="example">
		For example, the following selector matches all <a element>button</a> elements in an HTML document
		that are not disabled.

		<pre>button:not(\[DISABLED]) </pre>

		The following selector represents all but FOO elements.

		<pre>*:not(FOO)</pre>

		The following compound selector represents all HTML elements
		except links.

		<pre>html|*:not(:link):not(:visited)</pre>
	</div>

	As with '':is()'',
	default namespace declarations do not affect the <a>compound selector</a>
	representing the <a for=selector>subject</a> of any selector
	within a '':not()'' pseudo-class,
	unless that compound selector contains
	an explicit <a>universal selector</a> or <a>type selector</a>.
	(See '':is()'' for examples.)

	Note: The '':not()'' pseudo-class allows useless selectors to be written.
	For instance '':not(*|*)'', which represents no element at all,
	or ''div:not(span)'', which is equivalent to ''div'' but with a higher specificity.


<h3 id="zero-matches">
The Specificity-adjustment Pseudo-class: '':where()''</h3>

	<wpt>
		invalidation/where.html
		parsing/parse-where.html
		pseudo-where-crash.html
	</wpt>

	The Specificity-adjustment pseudo-class, <dfn id="where-pseudo">:where()</dfn>,
	is a functional pseudo-class
	with the same syntax and functionality as '':is()''.
	Unlike '':is()'', neither the '':where()'' pseudo-class, nor any of its arguments,
	contribute to the [=specificity=] of the selector--
	its [=specificity=] is always zero.

	This is useful for introducing filters in a selector
	while keeping the associated style declarations easy to override.

	<div class="example">
		Below is a common example where the specificity heuristic fails
		to match author expectations:

		<pre>
			a:not(:hover) {
				text-decoration: none;
			}

			nav a {
				/* Has no effect */
				text-decoration: underline;
			}
		</pre>

		However, by using '':where()'' the author can explicitly declare their intent:

		<pre>
			a:where(:not(:hover)) {
				text-decoration: none;
			}

			nav a {
				/* Works now! */
				text-decoration: underline;
			}
		</pre>
	</div>

	Note: Future levels of Selectors may introduce an additional argument
	to explicitly set the specificity of that instance of the pseudo-class.


<h3 id='relational'>
The Relational Pseudo-class: '':has()''</h3>

	<wpt>
		has-argument-with-explicit-scope.html
		has-basic.html
		has-display-none-checked.html
		has-focus-display-change.html
		has-matches-to-uninserted-elements.html
		has-nth-of-crash.html
		has-relative-argument.html
		has-sibling-chrome-crash.html
		has-specificity.html
		has-style-sharing-001.html
		has-style-sharing-002.html
		has-style-sharing-003.html
		has-style-sharing-004.html
		has-style-sharing-005.html
		has-style-sharing-006.html
		has-style-sharing-007.html
		has-style-sharing-pseudo-001.html
		has-style-sharing-pseudo-002.html
		has-style-sharing-pseudo-003.html
		has-style-sharing-pseudo-004.html
		has-style-sharing-pseudo-005.html
		has-style-sharing-pseudo-006.html
		has-style-sharing-pseudo-007.html
		has-style-sharing-pseudo-008.html
		has-visited.html
		invalidation/attribute-or-elemental-selectors-in-has.html
		invalidation/child-indexed-pseudo-classes-in-has.html
		invalidation/crashtests/has-pseudoclass-only-crash.html
		invalidation/defined-in-has.html
		invalidation/dir-pseudo-class-in-has.html
		invalidation/empty-pseudo-in-has.html
		invalidation/fullscreen-pseudo-class-in-has.html
		invalidation/has-append-first-node.html
		invalidation/has-complexity.html
		invalidation/has-css-nesting-shared.html
		invalidation/has-in-adjacent-position.html
		invalidation/has-in-ancestor-position.html
		invalidation/has-in-parent-position.html
		invalidation/has-in-sibling-position.html
		invalidation/has-invalidation-after-removing-non-first-element.html
		invalidation/has-invalidation-first-in-sibling-chain.html
		invalidation/has-invalidation-for-wiping-an-element.html
		invalidation/has-nested-pseudo-001-crash.html
		invalidation/has-nested-pseudo-002-crash.html
		invalidation/has-nested-pseudo-003-crash.html
		invalidation/has-pseudo-element.html
		invalidation/has-pseudoclass-only.html
		invalidation/has-sibling-insertion-removal.html
		invalidation/has-sibling.html
		invalidation/has-side-effect.html
		invalidation/has-unstyled.html
		invalidation/has-with-is-child-combinator.html
		invalidation/has-with-nesting-parent-containing-complex.html
		invalidation/has-with-nesting-parent-containing-hover.html
		invalidation/has-with-not.html
		invalidation/has-with-nth-child-sibling-remove.html
		invalidation/has-with-nth-child.html
		invalidation/has-with-pseudo-class.html
		invalidation/host-context-pseudo-class-in-has.html
		invalidation/host-has-shadow-tree-element-at-nonsubject-position.html
		invalidation/host-has-shadow-tree-element-at-subject-position.html
		invalidation/host-pseudo-class-in-has.html
		invalidation/input-pseudo-classes-in-has.html
		invalidation/is-pseudo-containing-complex-in-has.html
		invalidation/is-pseudo-containing-sibling-relationship-in-has.html
		invalidation/lang-pseudo-class-in-has-document-element.html
		invalidation/lang-pseudo-class-in-has-multiple-document-elements.html
		invalidation/lang-pseudo-class-in-has-xhtml.xhtml
		invalidation/lang-pseudo-class-in-has.html
		invalidation/link-pseudo-class-in-has.html
		invalidation/link-pseudo-in-has.html
		invalidation/location-pseudo-classes-in-has.html
		invalidation/media-loading-pseudo-classes-in-has.html
		invalidation/media-pseudo-classes-in-has.html
		invalidation/modal-pseudo-class-in-has.html
		invalidation/negated-has-in-nonsubject-position.html
		invalidation/not-pseudo-containing-complex-in-has.html
		invalidation/not-pseudo-containing-sibling-relationship-in-has.html
		invalidation/open-pseudo-class-in-has.html
		invalidation/state-in-has.html
		invalidation/subject-has-invalidation-with-display-none-anchor-element.html
		invalidation/target-pseudo-in-has.html
		invalidation/typed-child-indexed-pseudo-classes-in-has.html
		invalidation/user-action-pseudo-classes-in-has.html
		parsing/parse-has-disallow-nesting-has-inside-has.html
		parsing/parse-has-forgiving-selector.html
		parsing/parse-has.html
	</wpt>

	The relational pseudo-class, <dfn id='has-pseudo'>:has()</dfn>,
	is a functional pseudo-class taking a <<relative-selector-list>> as an argument.
	It represents an element if any of the <a>relative selectors</a>
	would match at least one element
	when [=anchor element|anchored against=] this element.

	The '':has()'' pseudo-class cannot be nested;
	'':has()'' is not valid within '':has()''.
	Also, unless explicitly defined as a
	<dfn dfn export>:has-allowed pseudo-element</dfn>,
	[=pseudo-elements=] are not valid selectors within '':has()''.
	(This specification does not define any [=:has-allowed pseudo-elements=],
	but other specifications may do so.)

	Note: Pseudo-elements are generally excluded from '':has()'' because
	many of them exist conditionally, based on the styling of their ancestors,
	so allowing these to be queried by '':has()'' would introduce cycles.

	Note: Since '':has()'' takes a <<relative-selector-list>>,
	its arguments are <em>inherently</em> [=complex selectors=]
	(because they start, perhaps implicitly, with a combinator).
	This means '':has()'' cannot be used in contexts that don't allow complex selectors;
	its arguments will be guaranteed to be invalid.

	<div class='example'>
		For example, the following selector matches only
		<code>&lt;a></code> elements that contain an <code>&lt;img></code> child:

		<pre>a:has(> img)</pre>

		The following selector matches a <code>&lt;dt></code> element
		immediately followed by another <code>&lt;dt></code> element:

		<pre>dt:has(+ dt)</pre>

		The following selector matches <code>&lt;section></code> elements
		that don't contain any heading elements:

		<pre>section:not(:has(h1, h2, h3, h4, h5, h6))</pre>

		Note that ordering matters in the above selector.
		Swapping the nesting of the two pseudo-classes, like:

		<pre>section:has(:not(h1, h2, h3, h4, h5, h6))</pre>

		...would result in matching any <code>&lt;section></code> element
		which contains anything that's not a heading element.
	</div>


<h2 id="elemental-selectors">
Elemental selectors</h2>

<h3 id="type-selectors">
Type (tag name) selector</h3>

	A <dfn export>type selector</dfn> is the name of a document language element type,
	and represents an instance of that element type in the document tree.

	<div class="example">
		For example, the selector ''h1'' represents an <a element>h1</a> element in the document.
	</div>

	A <a>type selector</a> is written as a <a>CSS qualified name</a>:
	an <a>identifier</a> with an optional namespace prefix.
	[[!CSS3NAMESPACE]]
	(See [[#type-nmsp]].)


<h3 id="the-universal-selector">
Universal selector </h3>

	<wpt>
		parsing/parse-universal.html
	</wpt>

	The <dfn export>universal selector</dfn> is a special <a>type selector</a>,
	that represents an element of any element type.

	It is written as a <a>CSS qualified name</a>
	with an asterisk (<code>*</code> U+002A) as the local name.
	Like a <a>type selector</a>,
	the <a>universal selector</a> can be qualified by a namespace,
	restricting it to only elements belonging to that namespace,
	and is affected by a default namespace as defined in [[#type-nmsp]].

	Unless an element is <a>featureless</a>,
	the presence of a <a>universal selector</a> has no effect on whether the element matches the selector.
	(<a>Featureless</a> elements do not match any selector,
		including the <a>universal selector</a>.)

	<div class="example">
		<ul>
			<li>
				<css>*[hreflang|=en]</css> and <css>[hreflang|=en]</css> are equivalent,
			<li>''*.warning'' and ''.warning'' are equivalent,
			<li>''*#myid'' and ''#myid'' are equivalent.
		</ul>
	</div>

	The <a>universal selector</a> follows the same syntax rules as other <a>type selectors</a>:
	only one can appear per <a>compound selector</a>,
	and it must be the first <a>simple selector</a> in the <a>compound selector</a>.

	Note: In some cases, adding a <a>universal selector</a> can make a selector easier to read,
	even though it has no effect on the matching behavior.
	For example, ''div :first-child'' and ''div:first-child'' are somewhat difficult to tell apart at a quick glance,
	but writing the former as ''div *:first-child'' makes the difference obvious.


<h3 id='type-nmsp'>
Namespaces in Elemental Selectors</h3>

	<a>Type selectors</a> and <a>universal selectors</a> allow an optional namespace component:
	a namespace prefix that has been previously <a href="#nsdecl">declared</a>
	may be prepended to the element name separated by the namespace separator “vertical bar” (<code>|</code> U+007C).
	(See, e.g., [[XML-NAMES]] for the use of namespaces in XML.)
	It has the following meaning in each form:

	<dl>
		<dt><code>ns|E</code>
		<dd>
			elements with name E in namespace ns

		<dt><code>*|E</code>
		<dd>
			elements with name E in any namespace,
			including those without a namespace

		<dt><code>|E</code>
		<dd>
			elements with name E without a namespace

		<dt><code>E</code>
		<dd>
			if no default namespace has been <a href="#nsdecl">declared</a> for selectors,
			this is equivalent to *|E.
			Otherwise it is equivalent to ns|E
			where ns is the default namespace.
	</dl>

	<div class="example">
		CSS examples:

		<pre>
		@namespace foo url(https://umn0mtkzgjkmem4kvumj8.julianrbryant.com);
		foo|h1 { color: blue }  /* first rule */
		foo|* { color: yellow } /* second rule */
		|h1 { color: red }      /* ...*/
		*|h1 { color: green }
		h1 { color: green }
		</pre>

		The first rule (not counting the ''@namespace'' at-rule)
		will match only <a element>h1</a> elements in the
		"https://umn0mtkzgjkmem4kvumj8.julianrbryant.com" namespace.

		The second rule will match all elements in the
		"https://umn0mtkzgjkmem4kvumj8.julianrbryant.com" namespace.

		The third rule will match only <a element>h1</a> elements with
		no namespace.

		The fourth rule will match <a element>h1</a> elements in any
		namespace (including those without any namespace).

		The last rule is equivalent to the fourth rule because no default
		namespace has been defined.
	</div>

	If a <a>default namespace</a> is declared,
	<a>compound selectors</a> without <a>type selectors</a> in them
	still only match elements in that default namespace.

	<div class='example'>
		For example,
		in the following style sheet:

		<pre>
		@namespace url("https://umn4kq9urycyna8.julianrbryant.com/foo");

		.special { ... }
		</pre>

		The ''.special'' selector only matches elements in the "https://umn4kq9urycyna8.julianrbryant.com/foo" namespace,
		even though no reference to the type name (which is paired with the namespace in the DOM) appeared.
	</div>

	A <a>type selector</a> or <a>universal selector</a> containing a namespace prefix
	that has not been previously <a href="#nsdecl">declared</a>
	is an <a>invalid selector</a>.


<h3 id="the-defined-pseudo">
The Defined Pseudo-class: '':defined''</h3>

	<wpt>
		invalidation/defined.html
	</wpt>

	In some host languages,
	elements can have a distinction between being “defined”/“constructed” or not.
	The <dfn id='defined-pseudo'>:defined</dfn> <a>pseudo-class</a> matches elements
	that are fully defined,
	as dictated by the host language.

	If the host language does not have this sort of distinction,
	all elements in it match '':defined''.

	<div class="example">
		In HTML, all built-in elements are always considered to be defined,
		so the following example will always match:

		<pre highlight=css>p:defined { ... }</pre>

		[=Custom elements=], on the other hand,
		start out <em>un</em>defined,
		and only become defined when <l spec=html>[=element definition|properly registered=]</l>.
		This means the '':defined'' pseudo-class
		can be used to hide a custom element
		until it has been registered:

		<pre>custom-element { visibility: hidden }</pre>
		<pre>custom-element:defined { visibility: visible }</pre>
	</div>


<h2 id="attribute-selectors">
Attribute selectors</h2>

	<wpt>
		attribute-selectors/style-attribute-selector.html
		invalidation/attribute.html
		invalidation/class-id-attr.html
		missing-right-token.html
		parsing/parse-attribute.html
		selectors-attr-many.html
	</wpt>

	Selectors allow the representation of an element's attributes. When
	a selector is used as an expression to match against an element,
	an <dfn export>attribute selector</dfn> must be considered to match an element if that
	element has an attribute that matches the attribute represented by the
	attribute selector.

	<p class="issue">Add comma-separated syntax for
	<a href="https://umndruhmgkj46tygt32g.julianrbryant.com/Archives/Public/www-style/2011Mar/0215.html">multiple-value matching</a>?
	e.g. [rel ~= next, prev, up, first, last]


<h3 id="attribute-representation">
Attribute presence and value selectors</h3>

	CSS2 introduced four attribute selectors:

	<dl>
		<dt>''[att]''
		<dd>
			Represents an element with the <code>att</code> attribute,
			whatever the value of the attribute.

		<dt><css>[att=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value is exactly "val".

		<dt><css>[att~=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value is a <a href="#whitespace">whitespace</a>-separated list of words,
			one of which is exactly "val".
			If "val" contains whitespace,
			it will never represent anything
			(since the words are <em>separated</em> by spaces).
			Also if "val" is the empty string,
			it will never represent anything.

		<dt><css>[att|=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute,
			its value either being exactly "val"
			or beginning with "val" immediately followed by "-" (U+002D).
			This is primarily intended to allow language subcode matches
			(e.g., the <code>hreflang</code> attribute on the <a element>a</a> element in HTML)
			as described in BCP 47 ([[BCP47]]).
			For <code>lang</code> (or <code>xml:lang</code>) language subcode matching,
			please see the '':lang()'' pseudo-class.
	</dl>

	Attribute values must be <<ident-token>>s or <<string-token>>s. [[!CSS3SYN]]

	<div class="example">
		Examples:

		The following attribute selector represents an <a element>h1</a> element
		that carries the <code>title</code> attribute,
		whatever its value:

		<pre>h1[title]</pre>

		In the following example, the selector represents a
		<a element>span</a> element whose <code>class</code> attribute has
		exactly the value "example":

		<pre>span[class="example"]</pre>

		Multiple attribute selectors can be used to represent several
		attributes of an element, or several conditions on the same
		attribute. Here, the selector represents a <a element>span</a> element
		whose <code>hello</code> attribute has exactly the value "Cleveland"
		and whose <code>goodbye</code> attribute has exactly the value
		"Columbus":

		<pre>span[hello="Cleveland"][goodbye="Columbus"]</pre>

		The following CSS rules illustrate the differences between
		"=" and "~=".  The first selector would match, for example, an
		<a element>a</a> element with the value "copyright copyleft
		copyeditor" on a <code>rel</code> attribute. The second selector
		would only match an <a element>a</a> element with an <code>href</code>
		attribute having the exact value "https://umn0mtkzgkj46tygt32g.julianrbryant.com/".

		<pre>
		a[rel~="copyright"] { ... }
		a[href="https://umn0mtkzgkj46tygt32g.julianrbryant.com/"] { ... }
		</pre>

		The following selector represents an <a element>a</a> element
		whose <code>hreflang</code> attribute is exactly "fr".

		<pre>a[hreflang=fr] </pre>

		The following selector represents an <a element>a</a> element for
		which the value of the <code>hreflang</code> attribute begins with
		"en", including "en", "en-US", and "en-scouse":

		<pre>a[hreflang|="en"] </pre>

		The following selectors represent a <a element lt=''>DIALOGUE</a> element
		whenever it has one of two different values for an attribute
		<code>character</code>:

		<pre>
		DIALOGUE[character=romeo]
		DIALOGUE[character=juliet]
		</pre>

	</div>


<h3 id="attribute-substrings">
Substring matching attribute selectors</h3>

	Three additional attribute selectors are provided for matching
	substrings in the value of an attribute:

	<dl>
		<dt><css>[att^=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value begins with the prefix "val".
			If "val" is the empty string
			then the selector does not represent anything.

		<dt><css>[att$=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value ends with the suffix "val".
			If "val" is the empty string
			then the selector does not represent anything.

		<dt><css>[att*=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value contains at least one instance of the substring "val".
			If "val" is the empty string
			then the selector does not represent anything.
	</dl>

	Attribute values must be <<ident-token>>s or <<string-token>>s.

	<div class="example">
		Examples:
		The following selector represents an HTML <a element>object</a> element,
		referencing an image:

		<pre>object[type^="image/"] </pre>

		The following selector represents an HTML <a element>a</a> element
		with an <code>href</code> attribute whose value ends with ".html".

		<pre>a[href$=".html"] </pre>

		The following selector represents an HTML paragraph
		with a <code>title</code> attribute whose value contains the substring "hello"

		<pre>p[title*="hello"] </pre>
	</div>


<h3 id="attribute-case">
Case-sensitivity</h3>

	<wpt>
		attribute-selectors/attribute-case/cssom.html
		attribute-selectors/attribute-case/semantics.html
		attribute-selectors/attribute-case/syntax.html
	</wpt>

	By default case-sensitivity of attribute names and values in selectors
	depends on the document language.

	To match attribute values [=ASCII case-insensitively=]
	regardless of document language rules,
	the attribute selector may include the identifier <code>i</code>
	before the closing bracket (<code>]</code>).
	When this flag is present,
	UAs must match the attribute's value
	[=ASCII case-insensitively=]
	(i.e. [a-z] and \[A-Z] are considered equivalent).

	<div class="example">
		For example, ASCII case insensitivity allows <code>green</code>
		to match <code>GREEN</code>.
		However, <code>grün</code> 
		would not match <code>GRÜN</code>.
	</div>

	Alternately, the attribute selector may include the identifier <code>s</code>
	before the closing bracket (<code>]</code>);
	in this case the UA must match the value case-sensitively,
	with “[=identical to=]” semantics [[INFRA]],
	regardless of document language rules.

	Like the rest of Selectors syntax,
	the <code>i</code> and <code>s</code> identifiers themselves
	are [=ASCII case-insensitive=].

	<div class="example">
		The following rule will style the <code>frame</code> attribute when it
		has a value of <code>hsides</code>, whether that value is represented
		as <code>hsides</code>, <code>HSIDES</code>, <code>hSides</code>, etc.
		even in an XML environment where attribute values are case-sensitive.

		<pre>[frame=hsides i] { border-style: solid none; } </pre>
	</div>

	<div class="example">
		The following rule will style lists with <code>type="a"</code>
		attributes differently than <code>type="A"</code>
		even though HTML defines the <code>type</code> attribute
		to be case-insensitive.

		<pre>
			[type="a" s] { list-style: lower-alpha; }
			[type="A" s] { list-style: upper-alpha; }
		</pre>
	</div>

	<!-- plinss notes we may eventually want to choose other normalizations
			 for attribute matching; but since there's no normalization scheme
			 that really seems needed at this point, this issue is deferred to
			 a later level -->

	Note: Some document models normalize case-insensitive attribute values at parse time
	such that checking if a string is case-sensitive matching is impossible.
	Case-sensitive matching via <code>s</code> flags is only possible
	in systems that preserve the original case.

<h3 id="attrnmsp">
Attribute selectors and namespaces</h3>

	<wpt>
		selectors-namespace-001.xml
	</wpt>

	The attribute name in an attribute selector is given as a
	[=CSS qualified name=]: a namespace prefix that has been previously [=declared=]
	may be prepended to the attribute name separated by the namespace
	separator &quot;vertical bar&quot; (<code>|</code>). In keeping with
	the Namespaces in the XML recommendation, default namespaces do not
	apply to attributes, therefore attribute selectors without a namespace
	component apply only to attributes that have no namespace (equivalent
	to ''|attr''). An asterisk may be used for
	the namespace prefix indicating that the selector is to match all
	attribute names without regard to the attribute's namespace.

	An attribute selector with an attribute name containing a namespace
	prefix that has not been previously [=declared=] is
	an invalid selector.

	<div class="example">
		CSS examples:

		<pre>
		@namespace foo "https://umn0mtkzgjkmem4kvumj8.julianrbryant.com";
		[foo|att=val] { color: blue }
		[*|att] { color: yellow }
		[|att] { color: green }
		[att] { color: green }
		</pre>

		The first rule will match only elements with the attribute
		<code>att</code> in the "https://umn0mtkzgjkmem4kvumj8.julianrbryant.com" namespace with the
		value "val".

		The second rule will match only elements with the attribute
		<code>att</code> regardless of the namespace of the attribute
		(including no namespace).

		The last two rules are equivalent and will match only elements
		with the attribute <code>att</code> where the attribute is not
		in a namespace.
	</div>


<h3 id="def-values">
Default attribute values in DTDs</h3>

	Attribute selectors represent attribute values in the document tree.
	How that document tree is constructed is outside the scope of Selectors.
	In some document formats default attribute values can be defined in a DTD or
	elsewhere, but these can only be selected by attribute selectors if they
	appear in the document tree. Selectors should be designed so that they
	work whether or not the default values are included in the document tree.

	For example, a XML UA may, but is <em>not</em> required to,
	read an “external subset” of the DTD, but <em>is</em> required to
	look for default attribute values in the document's “internal subset”.
	(See, e.g., [[XML10]] for definitions of these subsets.)
	Depending on the UA, a default attribute value defined in the external subset of the DTD
	might or might not appear in the document tree.

	A UA that recognizes an XML namespace may, but is not required to use its
	knowledge of that namespace to treat default attribute values as if
	they were present in the document. (For example, an XHTML UA is not
	required to use its built-in knowledge of the XHTML DTD. See, e.g., [[XML-NAMES]] for details on namespaces in XML
	1.0.)

	Note: Typically, implementations
	choose to ignore external subsets. This corresponds to the behavior
	of non-validating processors as defined by the XML specification.

	<div class="example">
		Example:

		Consider an element <code>EXAMPLE</code> with an attribute <code>radix</code>
		that has a default value of <code>"decimal"</code>. The DTD fragment might be

		<pre class="dtd-example">&lt;!ATTLIST EXAMPLE radix (decimal,octal) "decimal"> </pre>

		If the style sheet contains the rules

		<pre>
		EXAMPLE[radix=decimal] { /*... default property settings ...*/ }
		EXAMPLE[radix=octal]   { /*... other settings...*/ }
		</pre>

		the first rule might not match elements whose <code>radix</code> attribute is
		set by default, i.e. not set explicitly. To catch all cases, the
		attribute selector for the default value must be dropped:

		<pre>
		EXAMPLE                { /*... default property settings ...*/ }
		EXAMPLE[radix=octal]   { /*... other settings...*/ }
		</pre>

		Here, because the selector ''EXAMPLE[radix=octal]'' is
		more specific than the type selector alone, the style declarations in
		the second rule will override those in the first for elements that
		have a <code>radix</code> attribute value of <code>"octal"</code>. Care has to be taken that
		all property declarations that are to apply only to the default case
		are overridden in the non-default cases' style rules.
	</div>


<h3 id="class-html">
Class selectors</h3>

	<wpt>
		parsing/parse-class.html
		xml-class-selector.xml
	</wpt>

	The <dfn export>class selector</dfn> is given as a full stop (. U+002E) immediately
	followed by an identifier. It represents an element belonging to the
	class identified by the identifier, as defined by the document language.
	For example, in [[HTML5]], [[SVG11]], and [[MATHML]] membership in a
	class is given by the <code>class</code> attribute: in these languages
	it is equivalent to the <code>~=</code> notation applied to the
	local <code>class</code> attribute
	(i.e. <code>[class~=<var>identifier</var>]</code>).

	<div class="example">
		CSS examples:

		We can assign style information to all elements with
		<code>class~="pastoral"</code> as follows:

		<pre>*.pastoral { color: green }  /* all elements with class~=pastoral */ </pre>

		or just

		<pre>.pastoral { color: green }  /* all elements with class~=pastoral */ </pre>

		The following assigns style only to H1 elements with
		<code>class~="pastoral"</code>:

		<pre>H1.pastoral { color: green }  /* H1 elements with class~=pastoral */ </pre>

		Given these rules, the first <code>H1</code> instance below would not have
		green text, while the second would:

		<pre>
		&lt;H1>Not green&lt;/H1>
		&lt;H1 class="pastoral">Very green&lt;/H1>
		</pre>

		The following rule matches any <a element>P</a> element whose <code>class</code>
		attribute has been assigned a list of <a
		href="#whitespace">whitespace</a>-separated values that includes both
		<code>pastoral</code> and <code>marine</code>:

		<pre>p.pastoral.marine { color: green } </pre>

		This rule matches when <code>class="pastoral blue aqua
		marine"</code> but does not match for <code>class="pastoral
		blue"</code>.
	</div>

	Note: Because CSS gives considerable
	power to the "class" attribute, authors could conceivably design their
	own "document language" based on elements with almost no associated
	presentation (such as <a element>div</a> and <a element>span</a> in HTML)
	and assigning style
	information through the "class" attribute.  Authors should avoid this
	practice since the structural elements of a document language often
	have recognized and accepted meanings and author-defined classes may
	not.

	Note: If an element has multiple
	class attributes, their values must be concatenated with spaces
	between the values before searching for the class. As of this time the
	working group is not aware of any manner in which this situation can
	be reached, however, so this behavior is explicitly non-normative in
	this specification.

	When matching against a document which is in <a>quirks mode</a>,
	class names must be matched [=ASCII case-insensitively=];
	class selectors are otherwise case-sensitive,
	only matching class names they are [=identical to=]. [[INFRA]]


<h3 id="id-selectors">
ID selectors</h3>

	<wpt>
		historical-xmlid.xht
		parsing/parse-id.html
	</wpt>

	Document languages may contain attributes that are declared to be of type ID.
	What makes attributes of type ID special
	is that no two such attributes can have the same value in a conformant document,
	regardless of the type of the elements that carry them;
	whatever the document language,
	an ID typed attribute can be used to uniquely identify its element.
	In HTML all ID attributes are named <code>id</code>;
	XML applications may name ID attributes differently,
	but the same restriction applies.
	Which attribute on an element is considered the “ID attribute” is defined by the document language.

	An <dfn export>ID selector</dfn> consists of a “number sign” (U+0023, <code>#</code>)
	immediately followed by the ID value,
	which must be a CSS <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/CSS21/syndata.html#value-def-identifier">identifier</a>.
	An ID selector represents an element instance that has an identifier that matches the identifier in the ID selector.
	(It is possible in non-conforming documents for multiple elements to match a single ID selector.)

	<div class="example">
		Examples:
		The following ID selector represents an <a element>h1</a> element
		whose ID-typed attribute has the value "chapter1":

		<pre>h1#chapter1 </pre>

		The following ID selector represents any element whose ID-typed
		attribute has the value "chapter1":

		<pre>#chapter1 </pre>

		The following selector represents any element whose ID-typed
		attribute has the value "z98y".

		<pre>*#z98y </pre>
	</div>

	Note: In XML 1.0 [[XML10]], the information about which attribute
	contains an element's IDs is contained in a DTD or a schema. When
	parsing XML, UAs do not always read the DTD, and thus may not know
	what the ID of an element is (though a UA may have namespace-specific
	knowledge that allows it to determine which attribute is the ID
	attribute for that namespace). If a style sheet author knows or
	suspects that a UA may not know what the ID of an element is, they
	should use normal attribute selectors instead:
	''[name=p371]'' instead of ''#p371''.

	If an element has multiple ID attributes, all of them must be
	treated as IDs for that element for the purposes of the ID
	selector. Such a situation could be reached using mixtures of xml:id,
	DOM3 Core, XML DTDs, and namespace-specific knowledge.

	When matching against a document which is in <a>quirks mode</a>,
	IDs must be matched [=ASCII case-insensitively=];
	ID selectors are otherwise case-sensitive,
	only matching IDs they are [=identical to=]. [[INFRA]]


<h2 id="linguistic-pseudos">
Linguistic Pseudo-classes</h2>

<h3 id="the-dir-pseudo">
The Directionality Pseudo-class: '':dir()''</h3>

	<wpt>
		dir-pseudo-in-has.html
		dir-pseudo-on-bdi-element.html
		dir-pseudo-on-input-element.html
		dir-pseudo-update-document-element.html
		dir-selector-auto-direction-change-001.html
		dir-selector-auto.html
		dir-selector-change-001.html
		dir-selector-change-002.html
		dir-selector-change-003.html
		dir-selector-change-004.html
		dir-selector-ltr-001.html
		dir-selector-ltr-002.html
		dir-selector-ltr-003.html
		dir-selector-querySelector.html
		dir-selector-rtl-001.html
		dir-selector-white-space-001.html
		dir-style-01a.html
		dir-style-01b.html
		dir-style-02a.html
		dir-style-02b.html
		dir-style-03a.html
		dir-style-03b.html
		dir-style-04.html
		invalidation/part-dir.html
	</wpt>

	The <dfn id='dir-pseudo'>:dir()</dfn> pseudo-class allows the author to write
	selectors that represent an element based on its directionality
	as determined by the <a>document language</a>.
	For example, [[HTML5]] defines <a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/dom.html#the-directionality">how to determine the directionality of an element</a>,
	based on a combination of the <code>dir</code> attribute, the surrounding text, and other factors.
	As another example, the <code>its:dir</code> and <code>dirRule</code> element
	of the Internationalization Tag Set [[ITS20]]
	are able to define the directionality of an element in [[XML10]].

	The '':dir()'' pseudo-class does not select based on stylistic
	states&#8212;for example, the CSS 'direction' property does not affect
	whether it matches.

	The pseudo-class '':dir(ltr)'' represents an element that
	has a directionality of left-to-right (<code>ltr</code>). The
	pseudo-class '':dir(rtl)'' represents an element that has
	a directionality of right-to-left (<code>rtl</code>). The argument to
	'':dir()'' must be a single identifier, otherwise the selector
	is invalid. White space is optionally allowed between the identifier
	and the parentheses. Values other than <code>ltr</code> and
	<code>rtl</code> are not invalid, but do not match anything. (If a
	future markup spec defines other directionalities, then Selectors may
	be extended to allow corresponding values.)

	The difference between '':dir(C)'' and ''[dir=C]''
	is that ''[dir=C]'' only performs a comparison against a given
	attribute on the element, while the '':dir(C)'' pseudo-class
	uses the UAs knowledge of the document's semantics to perform the
	comparison. For example, in HTML, the directionality of an element
	inherits so that a child without a <code>dir</code> attribute will have
	the same directionality as its closest ancestor with a valid <code>dir</code>
	attribute. As another example, in HTML,
	an element that matches ''[dir=auto]'' will match either
	'':dir(ltr)'' or '':dir(rtl)'' depending on the resolved
	directionality of the elements as determined by its contents. [[HTML5]]


<h3 id="the-lang-pseudo">
The Language Pseudo-class: '':lang()''</h3>

	<wpt>
		i18n/css3-selectors-lang-001.html
		i18n/css3-selectors-lang-002.html
		i18n/css3-selectors-lang-004.html
		i18n/css3-selectors-lang-005.html
		i18n/css3-selectors-lang-006.html
		i18n/css3-selectors-lang-007.html
		i18n/css3-selectors-lang-008.html
		i18n/css3-selectors-lang-009.html
		i18n/css3-selectors-lang-010.html
		i18n/css3-selectors-lang-011.html
		i18n/css3-selectors-lang-012.html
		i18n/css3-selectors-lang-014.html
		i18n/css3-selectors-lang-015.html
		i18n/css3-selectors-lang-016.html
		i18n/css3-selectors-lang-021.html
		i18n/css3-selectors-lang-022.html
		i18n/css3-selectors-lang-024.html
		i18n/css3-selectors-lang-025.html
		i18n/css3-selectors-lang-026.html
		i18n/css3-selectors-lang-027.html
		i18n/css3-selectors-lang-028.html
		i18n/css3-selectors-lang-029.html
		i18n/css3-selectors-lang-030.html
		i18n/css3-selectors-lang-031.html
		i18n/css3-selectors-lang-032.html
		i18n/css3-selectors-lang-034.html
		i18n/css3-selectors-lang-035.html
		i18n/css3-selectors-lang-036.html
		i18n/css3-selectors-lang-041.html
		i18n/css3-selectors-lang-042.html
		i18n/css3-selectors-lang-044.html
		i18n/css3-selectors-lang-045.html
		i18n/css3-selectors-lang-046.html
		i18n/css3-selectors-lang-047.html
		i18n/css3-selectors-lang-048.html
		i18n/css3-selectors-lang-049.html
		i18n/css3-selectors-lang-050.html
		i18n/css3-selectors-lang-051.html
		i18n/css3-selectors-lang-052.html
		i18n/css3-selectors-lang-054.html
		i18n/css3-selectors-lang-055.html
		i18n/css3-selectors-lang-056.html
		i18n/lang-pseudo-class-across-shadow-boundaries.html
		i18n/lang-pseudo-class-disconnected.html
		i18n/lang-pseudo-class-empty-attribute.xhtml
		invalidation/part-lang.html
		selectors-4/lang-000.html
		selectors-4/lang-001.html
		selectors-4/lang-002.html
		selectors-4/lang-003.html
		selectors-4/lang-004.html
		selectors-4/lang-005.html
		selectors-4/lang-006.html
		selectors-4/lang-007.html
		selectors-4/lang-008.html
		selectors-4/lang-009.html
		selectors-4/lang-010.html
		selectors-4/lang-011.html
		selectors-4/lang-012.html
		selectors-4/lang-013.html
		selectors-4/lang-014.html
		selectors-4/lang-015.html
		selectors-4/lang-016.html
		selectors-4/lang-017.html
		selectors-4/lang-018.html
		selectors-4/lang-019.html
		selectors-4/lang-020.html
		selectors-4/lang-021.html
		selectors-4/lang-022.html
		selectors-4/lang-023.html
		selectors-4/lang-024.html
		selectors-4/lang-025.html
	</wpt>

	If the document language specifies how
	the (human) <a>content language</a> of an element is determined,
	it is possible to write selectors that
	represent an element based on its <a>content language</a>.
	The <dfn id='lang-pseudo'>:lang()</dfn> pseudo-class,
	which accepts a comma-separated list of one or more <a>language ranges</a>,
	represents an element whose [=content language=]
	is one of the languages listed in its argument.
	Each <dfn>language range</dfn> in '':lang()''
	is an <a href="https://un5gmtkzgjmt2j3jm38z69h0br.julianrbryant.com/rfc/rfc4647.html#section-2.2">extended language range</a>,
	as defined in BCP 47, and
	must be a valid CSS <<ident>> or <<string>>.
	(Thus language ranges containing asterisks, for example,
		must be either correctly escaped or quoted as strings,
		e.g. '':lang(\*-Latn)'' or '':lang("*-Latn")''.)

	Note: The <a>content language</a> of an element is defined by the document language.

	For example, in HTML [[HTML5]], the <a>content language</a> is determined
	by a combination of the <code>lang</code> attribute,
	information from <a element>meta</a> elements,
	and possibly also the protocol (e.g. from HTTP headers).
	XML languages can use the <code>xml:lang</code> attribute
	to indicate language information for an element. [[XML10]]

	The element's <a>content language</a> matches a <a>language range</a> if
	its <a>content language</a>, as represented in BCP 47 syntax,
	matches the given <a>language range</a> in an <i>extended filtering</i>
	operation per [[!RFC4647]] <cite>Matching of Language Tags</cite> (section 3.3.2).
	Both the [=content language=] and the [=language range=]
	must be <i>canonicalized</i>
	and converted to <i>extlang form</i> as per section 4.5 of [[!RFC5646]]
	prior to the <i>extended filtering</i> operation.
	The matching is performed [=ASCII case-insensitively=].

	The <a>language range</a> does not need to be a valid language code to
	perform this comparison.

	For this purpose, a wildcard [=language range=] (<code>"*"</code>)
	does not match elements whose language is not tagged (e.g. <code>lang=""</code>),
	but does match elements whose language is tagged as undetermined (<code>lang=und</code>).
	A [=language range=] consisting of an empty string
	('':lang("")'')
	matches (only) elements whose language is not tagged.

	Note: It is recommended that documents and protocols
	indicate language using language tags from [[BCP47]],
	and in the case of XML-based formats, by means of <code>xml:lang</code> attributes. [[XML10]]
	See <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/International/questions/qa-choosing-language-tags">“Choosing a Language Tag”</a>

	<div class="example">
		Examples:
		The two following selectors represent an HTML document that is in
		Belgian French or German. The two next selectors represent
		<a element>q</a> quotations in an arbitrary element in Belgian French
		or German.

		<pre>
		html:lang(fr-be)
		html:lang(de)
		:lang(fr-be) > q
		:lang(de) > q
		</pre>
	</div>

	Note: One difference between '':lang(C)'' and the ''|='' operator
	is that the ''|='' operator only performs a comparison against a given
	attribute on the element, while the '':lang(C)'' pseudo-class
	uses the UAs knowledge of the document's semantics to perform the
	comparison.

	<div class=example>
		In this HTML example, only the BODY matches
		''[lang|=fr]'' (because it has a LANG attribute) but both
		the BODY and the P match '':lang(fr)'' (because both are in
		French). The P does not match the ''[lang|=fr]'' because it
		does not have a LANG attribute.

		<pre>
		&lt;body lang=fr>
		  &lt;p>Je suis français.&lt;/p>
		&lt;/body>
		</pre>
	</div>

	<div class=example>
		Another difference between '':lang(C)'' and the ''|='' operator
		is that '':lang(C)'' performs implicit wildcard matching.

		For example, '':lang(de-DE)'' will match all of ''de-DE'',
		''de-DE-1996'', ''de-Latn-DE'', ''de-Latf-DE'', ''de-Latn-DE-1996'',
		whereas of those ''[lang|=de-DE]'' will only match ''de-DE'' and
		''de-DE-1996''.

		To perform wildcard matching on the first subtag (the primary language),
		an asterisk must be used: ''*-CH'' will match all of ''de-CH'',
		''it-CH'', ''fr-CH'', and ''rm-CH''.

		To select against an element’s lang attribute value
		using this type of language range match,
		use both the attribute selector and language pseudo-class together,
		e.g. ''[lang]:lang(de-DE)''.
	</div>

	Note: Wildcard language matching and comma-separated lists are new in Level 4.


<h2 id="location">
Location Pseudo-classes</h2>

<h3 id="the-any-link-pseudo">
The Hyperlink Pseudo-class: '':any-link''</h3>

	<wpt>
		invalidation/any-link-attribute-removal.html
		invalidation/any-link-pseudo.html
	</wpt>

	The <dfn id='any-link-pseudo'>:any-link</dfn> pseudo-class represents an element
	that acts as the source anchor of a hyperlink.
	For example, in [[HTML5]],
	any <{a}> or <{area}> elements with an <{a/href}> attribute are hyperlinks,
	and thus match <code>:any-link</code>.
	It matches an element if the element would match either '':link'' or '':visited'',
	and is equivalent to '':is(:link, :visited)''.


<h3 id="link">
The Link History Pseudo-classes: '':link'' and '':visited''</h3>

	<wpt>
		caret-color-visited-inheritance.html
		text-emphasis-visited-inheritance.html
		text-fill-color-visited-inheritance.html
		text-stroke-color-visited-inheritance.html
		visited-in-visited-compound.html
		visited-inheritance.html
		visited-nested.html
		visited-part-crash.html
	</wpt>

	User agents commonly display unvisited [[#the-any-link-pseudo|hyperlinks]]
	differently from previously visited ones.
	Selectors provides the pseudo-classes
	<dfn id='link-pseudo'>:link</dfn> and <dfn id='visited-pseudo'>:visited</dfn>
	to distinguish them:

	<ul>
		<li>The '':link'' pseudo-class applies to links that have
		not yet been visited.
		<li>The '':visited'' pseudo-class applies once the link has
		been visited by the user.
	</ul>

	The two states are mutually exclusive.

	After some amount of time,
	user agents may choose to return a visited link
	to the (unvisited) '':link'' state.

	The '':visited'' pseudo-class comes with obvious privacy implications--
	letting random websites know what <em>other</em> websites you've visited
	can be problematic for a number of reasons--
	and so user agents <em>must</em> preserve user privacy
	in their implementation of '':visited''.

	<div class=note>
		This specification intentionally does not specify
		exactly how to preserve user privacy in this regard,
		to allow for user agents to innovate in this space.
		The following methods are suggested, however:

		* Have '':visited'' never match,
			so all links match '':link'' instead.
		* Carefully track what history entries
			could have been observed by a given origin on their own,
			and only have links match '':visited''
			if that visit would have been observable from the site's origin.
			A possible specific approach for this
			is described in [[#visited-privacy]].
		* Allow links to match '':visited'' on any origin,
			but carefully restrict what styles they can apply
			and what information is returned by style-querying APIs
			like {{getComputedStyle()}},
			to prevent sites from observing
			whether a link is styled with '':link'' or '':visited''.
			(This is documented at <a href="https://un5j2j18xhuv2emkwgjjkgb49yug.julianrbryant.com/en-US/docs/Web/CSS/Privacy_and_the_:visited_selector">MDN</a>,
			and was the historical approach browsers took,
			but is not perfect;
			there are several ways for a hostile page
			to still extract history information.)
	</div>

	<div class="example">
		For example, the selector ''.footnote:visited''
		would allow styling footnote links differently
		if they've been previously followed,
		allowing users of the page to know
		they might not need to click the footnote again.
	</div>

<h3 id="the-target-pseudo">
The Target Pseudo-class: '':target''</h3>

	In some document languages,
	the document's URL can further point to specific elements <em>within</em> the document
	via the URL's <a for=url>fragment</a>.
	The elements pointed to in this way are the target elements of the document.

	<div class='example'>
		In HTML the fragment points to the element in the page with the same ID.
		The url <code>https://example.com/index.html#section2</code>,
		for example,
		points to the element with <code>id="section2"</code>
		in the document at <code>https://example.com/index.html</code>.
	</div>

	The <dfn id='target-pseudo'>:target</dfn> pseudo-class matches the document's target elements.
	If the document's URL has no fragment identifier, then the document has no target elements.

	<div class="example">
		Example:

		<pre>p.note:target </pre>

		This selector represents a <{p}> element of class
		<code>note</code> that is the target element of the referring
		URL.
	</div>

	<div class="example">
		CSS example:
		Here, the '':target'' pseudo-class is used to make the
		target element red and place an image before it, if there is one:

		<pre>
		:target { color : red }
		:target::before { content : url(target.png) }
		</pre>
	</div>

	Note: This specification previously defined a <css>:target-within</css> pseudo-class,
	analogous to '':focus-within''.
	It was removed in favor of '':has(:target)'',
	which should hopefully suffice to solve the same use-cases.


<h3 id="the-scope-pseudo">
The Reference Element Pseudo-class: '':scope''</h3>

	<wpt>
		scope-selector.html
		scope-without-scoping.html
	</wpt>

	In some contexts, selectors are matched
	with respect to one or more [=scoping roots=],
	such as when calling the {{Element/querySelector()}} method in [[DOM]].
	The <dfn id='scope-pseudo'>:scope</dfn> pseudo-class represents
	this [=scoping root=],
	and may be either a true element
	or a virtual one (such as a {{DocumentFragment}}).

	If there is no [=scoping root=]
	then '':scope'' represents the root of the tree the element is in
	(equivalent to '':host'' in a [=shadow tree=],
	or '':root'' otherwise).
	Specifications intending for this pseudo-class to match specific elements
	rather than this tree root element
	must define their [=scoping root|scoping root(s)=].

	A virtual [=scoping root=] is some object representing the root of a document fragment,
	and can be used in selector patterns to represent other elements’ relationships
	to this [=scoping root=],
	acting as the parent of any root elements in the document fragment it represents.
	A virtual [=scoping root=] is [=featureless=]
	and cannot be the [=subject of the selector=].

	<div class=example>
		For example, if you have a {{DocumentFragment}} <code>df</code>,
		then <code highlight=js>df.querySelectorAll(":scope > .foo")</code>
		matches all the ''.foo'' elements that are "top-level" in the document fragment
		(those that have the document fragment as their {{Node/parentNode}}).

		However, <code highlight=js>df.querySelector(":scope")</code> will not match anything,
		as the document fragment itself can't be the [=subject of the selector=].
	</div>


<h2 id="useraction-pseudos">
User Action Pseudo-classes</h2>

	Interactive user interfaces sometimes change the rendering in response to user actions.
	Selectors provides several <dfn lt="user action pseudo-class">user action pseudo-classes</dfn>
	for the selection of an element the user is acting on.
	(In non-interactive user agents, these pseudo-classes are valid, but never match any element.)

	The [=user action pseudo-classes=] are not mutually exclusive.
	An element can match several such pseudo-classes at the same time.

	<div class="example">
		Examples:

		<pre>
		a:hover   /* user hovers over the link */
		a:focus   /* user focuses the link     */

		a:focus:hover
		/* user hovers over the link while it's focused */
		</pre>
	</div>

	Some [=user action pseudo-classes=],
	in addition to matching on the particular element with the property in question,
	match on that element's ancestors as well:
	'':hover'', '':active'', '':focus-within''.
	Specifically, if these match on a given element,
	they also match on the element's [=flat tree=] ancestors,
	up to and including the first [=top layer=] element
	or the [=tree/root=] element,
	whichever is encountered first.

	Note: The specifics of hit-testing,
	necessary to know when several of the pseudo-classes defined in this section apply,
	are not yet defined,
	but will be in the future.

<h3 id="the-hover-pseudo">
The Pointer Hover Pseudo-class: '':hover''</h3>

	<wpt>
		hover-001-manual.html
		hover-002.html
		remove-hovered-element.html
	</wpt>

	The <dfn id='hover-pseudo'>:hover</dfn> pseudo-class applies
	while the user designates an element (or pseudo-element)
	with a pointing device,
	but does not necessarily activate it.
	For example, a visual user agent could apply this pseudo-class
	when the cursor (mouse pointer) hovers over a box generated by the element.
	Interactive user agents that cannot detect hovering due to hardware limitations
	(e.g., a pen device that does not detect hovering)
	are still conforming;
	the selector will simply never match in such a UA.

	An element also matches '':hover''
	if one of its descendants in the <a>flat tree</a>
	(including non-element nodes, such as text nodes)
	matches the above conditions.

	Document languages may define additional ways in which an element can match '':hover''.
	For example, [[HTML5]] defines a labeled control element as <a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selector-hover">matching <code>:hover</code></a>
	when its <a element>label</a> is hovered.

	Note: Since the '':hover'' state can apply to an element
	because its child is designated by a pointing device,
	it is possible for '':hover'' to apply
	to an element that is not underneath the pointing device.


<h3 id="the-active-pseudo">
The Activation Pseudo-class: '':active''</h3>

	The <dfn id='active-pseudo'>:active</dfn> pseudo-class
	applies while an element is being “activated” by the user,
	as defined by the host language;
	for example, while a hyperlink is being triggered.

	In addition, the '':active'' pseudo-class applies
	while any generated box of <em>any</em> element (or pseudo-element)
	is being actively indicated by a pointing device (in the “down” state),
	e.g. between the time the user presses the primary mouse button and releases it,
	or while a finger is pressing on a touchscreen.

	Note: [[HTML5]] defines
	<a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selector-active">
	specific conditions for HTML elements to be activated
	</a>.

	An element also matches '':active''
	if one of its descendants in the <a>flat tree</a>
	(including non-element nodes, such as text nodes)
	matches the above conditions.


<h3 id="the-focus-pseudo">
The Input Focus Pseudo-class: '':focus''</h3>

	<wpt>
		focus-display-none-001.html
		focus-in-focus-event-001.html
		focus-in-focusin-event-001.html
	</wpt>

	The <dfn id='focus-pseudo'>:focus</dfn> pseudo-class applies
	while an element (or pseudo-element) has the focus
	(accepts keyboard or other forms of input).

	There may be document language or implementation specific limits on
	which elements can acquire '':focus''.
	For example, [[HTML]] defines a list of <a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/interaction.html#focusable-area">focusable areas</a>.

	Document languages may define additional ways in which an element can match '':focus'',
	except that the '':focus'' pseudo-class must not automatically propagate to the parent element--
	see '':focus-within'' if matching on the parent is desired.
	('':focus'' may still apply to the parent element
	if made to propagate due to other mechanisms,
	but not merely due to being the parent.)

	ISSUE: There's a desire from authors to propagate '':focus''
	from a form control to its associated <{label}> element;
	the main objection seems to be implementation difficulty.
	See <a href="https://github.com/w3c/csswg-drafts/issues/397">CSSWG issue (CSS)</a>
	and <a href="https://github.com/whatwg/html/issues/1632">WHATWG issue (HTML)</a>.


<h3 id="the-focus-visible-pseudo" oldids="the-focusring-pseudo,focus-ring-pseudo">
The Focus-Indicated Pseudo-class: '':focus-visible''</h3>

	<wpt>
		focus-visible-001.html
		focus-visible-002.html
		focus-visible-003.html
		focus-visible-004.html
		focus-visible-005.html
		focus-visible-006.html
		focus-visible-007.html
		focus-visible-008.html
		focus-visible-009.html
		focus-visible-010.html
		focus-visible-011.html
		focus-visible-012.html
		focus-visible-013.html
		focus-visible-014.html
		focus-visible-015.html
		focus-visible-016.html
		focus-visible-017-2.html
		focus-visible-017.html
		focus-visible-018-2.html
		focus-visible-018.html
		focus-visible-019.html
		focus-visible-020.html
		focus-visible-021.html
		focus-visible-023.html
		focus-visible-024.html
		focus-visible-025.html
		focus-visible-026.html
		focus-visible-027.html
		focus-visible-028.html
		focus-visible-script-focus-001.html
		focus-visible-script-focus-004.html
		focus-visible-script-focus-005.html
		focus-visible-script-focus-008-b.html
		focus-visible-script-focus-008.html
		focus-visible-script-focus-009.html
		focus-visible-script-focus-010.html
		focus-visible-script-focus-011.html
		focus-visible-script-focus-012.html
		focus-visible-script-focus-013.html
		focus-visible-script-focus-014.html
		focus-visible-script-focus-015.html
		focus-visible-script-focus-018.html
		focus-visible-script-focus-019.html
		focus-visible-script-focus-020.html
		parsing/parse-focus-visible.html
	</wpt>

	While the '':focus'' [=pseudo-class=] always matches the currently-focused element,
	UAs only sometimes visibly <dfn>indicate focus</dfn>
	(such as by drawing a “focus ring”),
	instead using a variety of heuristics
	to visibly indicate the focus only when it would be most helpful to the user.
	The <dfn id="focus-visible-pseudo">:focus-visible</dfn> [=pseudo-class=] matches a focused element (or pseudo-element)
	in these situations only,
	allowing authors to change the appearance of the focus indicator
	without changing <em>when</em> a focus indicator appears.

	<div class="example">
		In this example,
		all focusable elements get a strong yellow outline on '':focus-visible'',
		and links get both a yellow outline and a yellow background on '':focus-visible''.
		These styles are consistent throughout the page and are easily visible
		due to their bold styling,
		but do not appear unless the user is likely to need to understand
		where page focus is.

		<pre highlight=css>
		:root {
			--focus-gold: #ffbf47;
		}

		:focus-visible  {
			outline: 3px solid var(--focus-gold);
		}

		a:focus-visible {
			background-color: var(--focus-gold);
		}
		</pre>
	</div>

	<div class=example>
		User agents can choose their own heuristics for when to [=indicate focus=];
		however, the following (non-normative) suggestions can be used as a starting point
		for when to [=indicate focus=] on the currently focused element:

		* If the user has expressed a preference
			(such as via a system preference or a browser setting)
			to always see a visible focus indicator,
			[=indicate focus=]
			regardless of any other factors.
			(Another option may be for the user agent to show its own focus indicator
			regardless of author styles.)
		* If the element which supports keyboard input
			(such as an <{input}> element,
			or any other element
			that would triggers a virtual keyboard to be shown on focus
			if a physical keyboard were not present),
			[=indicate focus=].
		* If the user interacts with the page via keyboard or some other non-pointing device,
			[=indicate focus=].
			(This means keyboard usage may change whether this pseudo-class matches
			even if it doesn't affect '':focus'').
		* If the user interacts with the page via a pointing device (mouse, touchscreen, etc.)
			and the focused element does not support keyboard input,
			don't [=indicate focus=].
		* If the previously-focused element [=indicated focus=],
			and a script causes focus to move elsewhere,
			[=indicate focus=] on the newly focused element.

			Conversely, if the previously-focused element did not [=indicate focus=],
			and a script causes focus to move elsewhere,
			don't [=indicate focus=] on the newly focused element.
		* If a newly-displayed element automatically gains focus
			(such as an action button in a freshly opened dialog),
			that element should [=indicate focus=].
	</div>

	User agents should also use '':focus-visible'' to specify the default focus style,
	so that authors using '':focus-visible'' will not also need to disable
	the default '':focus'' style.


<h3 id="the-focus-within-pseudo">
The Focus Container Pseudo-class: '':focus-within''</h3>

	<wpt>
		focus-within-001.html
		focus-within-002.html
		focus-within-003.html
		focus-within-004.html
		focus-within-005.html
		focus-within-006.html
		focus-within-007.html
		focus-within-008.html
		focus-within-009.html
		focus-within-010.html
		focus-within-011.html
		focus-within-012.html
		focus-within-013.html
		focus-within-display-none-001.html
		focus-within-focus-move.html
		focus-within-removal.html
		focus-within-shadow-001.html
		focus-within-shadow-002.html
		focus-within-shadow-003.html
		focus-within-shadow-004.html
		focus-within-shadow-005.html
		focus-within-shadow-006.html
	</wpt>

	The <dfn id='focus-within-pseudo'>:focus-within</dfn> pseudo-class
	applies to any element (or pseudo-element)
	for which the '':focus'' pseudo-class applies,
	as well as to an element (or pseudo-element)
	whose descendant in the <a>flat tree</a>
	(including non-element nodes, such as text nodes)
	matches the conditions for matching '':focus''.


<h2 id='resource-pseudos'>
Resource State Pseudo-classes</h2>

	The pseudo-classes in this section apply to elements that represent loaded resources,
	particularly images/videos,
	and allow authors to select them based on some quality of their state.

<h3 id="video-state">
Media Playback State: the '':playing'', '':paused'', and '':seeking'' pseudo-classes</h3>

	<wpt>
		media/media-playback-state.html
	</wpt>

	The <dfn>:playing</dfn> pseudo-class represents an element
	that is capable of being “played” or “paused”,
	when that element is “playing”.
	(This includes both when the element is explicitly playing,
	and when it's temporarily stopped for some reason not connected to user intent,
	but will automatically resume when that reason is resolved,
	such as a “buffering” or “stalled” state.)

	The <dfn>:paused</dfn> pseudo-class represents an element
	that is capable of being “played” or “paused”,
	when that element is “paused” (i.e. <em>not</em> ”playing”).
	(This includes both an explicit “paused” state,
	and other non-playing states like “loaded, hasn't been activated yet”, etc.)

	The <dfn>:seeking</dfn> pseudo-class represents an element
	that is capable of ”seeking”
	when that element is ”seeking”.
	(For the <{audio}> and <{video}> elements of HTML, see [[HTML#seeking]].)

<h3 id="media-loading-state">
Media Loading State: the '':buffering'' and '':stalled'' pseudo-classes</h3>

	<wpt>
		media/media-loading-state.html
		media/media-loading-state-timing.sub.html
		media/media-playback-state-timing.html
	</wpt>

	The <dfn>:buffering</dfn> pseudo-class represents an element
	that is capable of being “played” or “paused”,
	when that element cannot continue playing
	because it is actively attempting to obtain [=HTMLMediaElement/media data=]
	but has not yet obtained enough data to resume playback.
	(Note that the element is still considered to be “playing” when it is “buffering”.
	Whenever '':buffering'' matches an element,
	'':playing'' also matches the element.)

	The <dfn>:stalled</dfn> pseudo-class represents an element
	when that element cannot continue playing
	because it is actively attempting to obtain [=HTMLMediaElement/media data=]
	but it has failed to receive any data for some amount of time.
	For the <{audio}> and <{video}> elements of HTML,
	this amount of time is the [=media element stall timeout=]. [[HTML]]
	(Note that, like with the '':buffering'' pseudo-class,
	the element is still considered to be “playing” when it is “stalled”.
	Whenever '':stalled'' matches an element,
	'':playing'' also matches the element.)


<h3 id="sound-state">
Sound State: the '':muted'' and '':volume-locked'' pseudo-classes</h3>

	<wpt>
		media/sound-state.html
	</wpt>

	The <dfn>:muted</dfn> pseudo-class represents
	an element that is capable of making sound,
	but is currently “muted“ (forced silent).
	(For the <{audio}> and <{video}> elements of HTML, see [=muted=]. [[HTML]])

	The <dfn>:volume-locked</dfn> pseudo-class represents
	an element that is capable of making sound,
	and currently has its volume "locked"
	by the UA or the user,
	so the page author cannot change it.
	(For the <{audio}> and <{video}> elements of HTML,
	see the algorithm for setting the element's [=HTMLMediaElement/effective media volume=]. [[HTML]])


<h2 id="display-state-pseudos">
Element Display State Pseudo-classes</h2>

<h3 id="open-state">
Collapse State: the '':open'' pseudo-class</h3>

	<wpt>
		open-pseudo.html
		selectors-4/details-open-pseudo-001.html
		selectors-4/details-open-pseudo-002.html
		selectors-4/details-open-pseudo-003.html
	</wpt>
	<wpt pathprefix="/css/css-pseudo/">
		input-element-pseudo-open.optional.html
	</wpt>

	The <dfn>:open</dfn> pseudo-class represents an element
	that has both “open” and “closed” states,
	and which is currently in the “open” state.

	Exactly what “open” and “closed” mean is host-language specific,
	but exemplified by elements such as
	HTML's <{details}>, <{select}>, <{dialog}>, and <{input}> elements,
	all of which can be toggled “open” to display more content
	(or any content at all, in the case of <{dialog}>).

	Note: Being “open” or “closed” is a semantic state.
	An element not currently being displayed
	(for example, one that has ''visibility: collapse'',
	or belongs to a ''display: none'' subtree)
	can still be “open” and will match '':open''.

	Note: A ":closed" pseudo-class might be added in the future
	once the full set of things that support '':open'' is known.


<h3 id="popover-open-state">
Popover State: the '':popover-open'' pseudo-class</h3>

	<wpt>
		popover-open-with-has-sibling-selector.html
	</wpt>

	The <dfn>:popover-open</dfn> pseudo-class represents an element
	that has both “popover-showing” and “popover-hidden” states
	and which is currently in the “popover-showing” state.

	Note: This is distinct from '':open'' because
	an element can have element-specific open and closed states
	and also have separate popover-showing and popover-hidden states
	associated with being a popover.

	Exactly what “popover-showing” and “popover-hidden” states
	mean is host-language specific,
	but is exemplified by the presence of the HTML <{html-global/popover}> attribute
	and the associated <a>popover visibility state</a>.


<h3 id="modal-state">
Modal (Exclusive Interaction) State: the '':modal'' pseudo-class</h3>

	<wpt>
		modal-pseudo-class.html
	</wpt>

	The <dfn>:modal</dfn> pseudo-class represents
	an element which is in a state that excludes all interaction
	with elements outside it until it has been dismissed.
	Multiple elements can be '':modal'' simultaneously,
	with only one of them active (able to receive input).

	<div class="example">
		For example, the <{dialog}> element is '':modal''
		when opened with the {{HTMLDialogElement/showModal()}} API.
		Similarly, a '':fullscreen'' element is also '':modal''
		when opened with the {{requestFullscreen()}} API,
		since this prevents interaction with the rest of the page.
	</div>


<h3 id="fullscreen-state">
Fullscreen Presentation State: the '':fullscreen'' pseudo-class</h3>

	The <dfn>:fullscreen</dfn> pseudo-class represents
	an element which is displayed in a mode that
	takes up most (usually all) of the screen,
	such as that defined by the Fullscreen API. [[FULLSCREEN]]


<h3 id="pip-state">
Picture-in-Picture Presentation State: the '':picture-in-picture'' pseudo-class</h3>

	The <dfn>:picture-in-picture</dfn> pseudo-class represents
	an element which is displayed in a mode that
	takes up most (usually all) of the viewport,
	and whose viewport is confined to part of the screen
	while being displayed over other content,
	for example when using the Picture-in-Picture API. [[picture-in-picture]]


<h2 id='input-pseudos'>
The Input Pseudo-classes</h2>

	The pseudo-classes in this section mostly apply to elements that take user input,
	such as HTML's <a element>input</a> element.


<h3 id="input-states">
Input Control States</h3>

<h4 id="enableddisabled">
The '':enabled'' and '':disabled'' Pseudo-classes</h4>

	<wpt>
		invalidation/enabled-disabled.html
		pseudo-enabled-disabled.html
	</wpt>

	The <dfn id='enabled-pseudo'>:enabled</dfn> pseudo-class represents
	user interface elements that are in an enabled state;
	such elements must have a corresponding disabled state.

	Conversely, the <dfn id='disabled-pseudo'>:disabled</dfn> pseudo-class represents
	user interface elements that are in a disabled state;
	such elements must have a corresponding enabled state.

	What constitutes an enabled state, a disabled state, and a user interface
	element is host-language-dependent. In a typical document most elements will be
	neither '':enabled'' nor '':disabled''.
	For example, [[HTML5]] defines <a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selector-enabled">non-disabled interactive elements</a> to be '':enabled'',
	and any such elements that are <a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selector-enabled">explicitly disabled</a> to be '':disabled''.

	Note: CSS properties that might affect a user’s ability
	to interact with a given user interface element do not affect whether it
	matches '':enabled'' or '':disabled''; e.g., the
	'display' and 'visibility' properties have no effect
	on the enabled/disabled state of an element.

<h4 id="rw-pseudos">
The Mutability Pseudo-classes: '':read-only'' and '':read-write''</h4>

	<wpt>
		selector-read-write-type-change-001.html
		selector-read-write-type-change-002.html
	</wpt>

	An element matches <dfn id="read-write-pseudo">:read-write</dfn> if it is user-alterable,
	as defined by the document language.
	Otherwise, it is <dfn id="read-only-pseudo">:read-only</dfn>.

	For example, in [[HTML5]] a <a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selector-read-only">non-disabled non-readonly <code>&lt;input></code> element</a> is '':read-write'',
	as is any element with the <code>contenteditable</code> attribute set to the true state.

<h4 id="placeholder">
The Placeholder-shown Pseudo-class: '':placeholder-shown''</h4>

	<wpt>
		invalidation/placeholder-shown.html
		placeholder-shown.html
		selector-placeholder-shown-emptify-placeholder.html
		selector-placeholder-shown-type-change-001.html
		selector-placeholder-shown-type-change-002.html
		selector-placeholder-shown-type-change-003.html
	</wpt>

	Input elements can sometimes show placeholder text
	as a hint to the user on what to type in.
	See, for example, the <code>placeholder</code> attribute in [[HTML5]].
	The <dfn id="placeholder-shown-pseudo">:placeholder-shown</dfn> pseudo-class
	matches an input element that is showing such placeholder text,
	whether that text is given by an attribute or a real element,
	or is otherwise implied by the UA.

	<div class="example">
		For example, according to the semantics of [[HTML]]
		the <{input/placeholder}> attribute on the <{input}> and <{textarea}> elements
		provide placeholder text.
		The '':placeholder-shown'' class thus applies
		whenever such placeholder text is shown.
	</div>

<h4 id="autofill">
The Automatic Input Pseudo-class: '':autofill''</h4>

	The <dfn>:autofill</dfn> pseudo-class represents input elements
	that have been automatically filled by the user agent,
	and have not been subsequently altered by the user.

<h4 id="the-default-pseudo">
The Default-option Pseudo-class: '':default''</h4>

	The <dfn id='default-pseudo'>:default</dfn> pseudo-class applies to the one or more UI elements
	that are the default among a set of similar elements.  Typically applies to
	context menu items, buttons and select lists/menus.

	One example is the default submit button among a set of buttons.
	Another example is the default option from a popup menu.
	In a select-many group (such as for pizza toppings), multiple elements can match '':default''.
	For example, [[HTML5]] defines that '':default'' matches
	<a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selector-default">the “default button” in a form,
	the initially-selected <code>&lt;option></code>(s) in a <code>&lt;select></code>,
	and a few other elements.</a>


<h3 id="input-value-states">
Input Value States</h3>

<h4 id="checked" oldids="indeterminate">
The Selected-option Pseudo-classes: '':checked'', '':unchecked'', and '':indeterminate''</h4>

	Radio and checkbox elements can be toggled by the user,
	and some menu items are “checked” when the user selects them.
	This state is reflected by the <dfn export>input value pseudo-classes</dfn>.

	When such elements are toggled “on”
	the <dfn id='checked-pseudo'>:checked</dfn> pseudo-class applies.
	For example, [[HTML5]] defines that <a href="https://un5nj90kzk5vf152hgyfw29h1eja2.julianrbryant.com/multipage/semantics-other.html#selector-checked">checked checkboxes, radio buttons, and selected <code>&lt;option></code> elements</a> match '':checked''.
	Similarly, when such elements are toggled “off”,
	the <dfn id='unchecked-pseudo'>:unchecked</dfn> pseudo-class applies.

	If an element that <em>could</em> match '':checked'' or '':unchecked''
	is neither "on" nor "off",
	the <dfn id='indeterminate-pseudo'>:indeterminate</dfn> pseudo-class applies.
	'':indeterminate'' also matches elements which do not have a notion of being "checked",
	but whose "value" is still in an indeterminate state,
	such as a progress meter whose progress percentage is unknown.

	These three pseudo-classes are mutually exclusive;
	an element can only match at most one of them at a time.
	(And might match none,
	if the concept of being "checked" doesn't apply to them.)
	If the "indeterminate" concept is not mutually exclusive
	with the "checked"/"unchecked" concept in a host language
	(for example, in HTML a checkbox's "indeterminate" state is a separate boolean
	from its "checked" state boolean),
	being indeterminate wins over being checked/unchecked
	for the purpose of matching these pseudo-classes.

	<div class=example>
		For example, in HTML:

		* Checkboxes match '':indeterminate''
			if their {{HTMLInputElement/indeterminate}} property is true;
			otherwise, they match '':checked'' or '':unchecked''
			depending on their {{HTMLInputElement/checked}} property.
		* Radio buttons match '':checked'' if their {{HTMLInputElement/checked}} property is true.
			Otherwise, they match '':unchecked'' if they're in a radio group
			that contains a '':checked'' radio button.
			Otherwise (their {{HTMLInputElement/checked}} property is false,
			but they're alone or in a radio group with no checked radio button),
			they match '':indeterminate''.
		* <{option}> elements within a <{select}>
			match '':checked'' or '':unchecked''
			depending on their {{HTMLOptionElement/selected}} property.
			(They're never '':indeterminate''.)
		* <{progress}> elements match '':indeterminate''
			if they lack a <code>value</code> attribute
	</div>

	While the [=input value pseudo-classes=] are dynamic in nature,
	and can altered by user action,
	since they can also be based on the presence of semantic attributes in the document
	(such as the <code>selected</code> and <code>checked</code> attributes in [[HTML5]]),
	they apply to all media.


<h3 id='ui-validity'>
Input Value-checking</h3>

<h4 id="validity-pseudos">
The Validity Pseudo-classes: '':valid'' and '':invalid''</h4>

	An element is <dfn id="valid-pseudo">:valid</dfn>
	or <dfn id="invalid-pseudo">:invalid</dfn>
	when its contents or value is, respectively,
	valid or invalid with respect to data validity semantics defined by the document language
	(e.g. [[XFORMS11]] or [[HTML5]]).
	An element which lacks data validity semantics is neither '':valid'' nor '':invalid''.

	Note: There is a difference between an element which has no constraints,
	and thus would always be '':valid'',
	and one which has no data validity semantics at all,
	and thus is neither '':valid'' nor '':invalid''.
	In HTML, for example, an <code>&lt;input type="text"></code> element may have no constraints,
	but a <a element>p</a> element has no validity semantics at all,
	and so it never matches either of these pseudo-classes.

<h4 id="range-pseudos">
The Range Pseudo-classes: '':in-range'' and '':out-of-range''</h4>

	The <dfn id="in-range-pseudo">:in-range</dfn> and
	<dfn id="out-of-range-pseudo">:out-of-range</dfn> pseudo-classes
	apply only to elements that have range limitations.  An element is
	'':in-range'' or '':out-of-range'' when the value
	that the element is bound to is in range or out of range with respect
	to its range limits as defined by the document language. An element
	that lacks data range limits or is not a form control is neither
	'':in-range'' nor '':out-of-range''.
	E.g. a slider element with a value of 11 presented as a slider control
	that only represents the values from 1-10 is :out-of-range.  Another
	example is a menu element with a value of "E" that happens to be
	presented in a popup menu that only has choices "A", "B" and "C".

<h4 id="opt-pseudos">
The Optionality Pseudo-classes: '':required'' and '':optional''</h4>

	<wpt>
		selector-required-type-change-001.html
		selector-required-type-change-002.html
		selector-required.html
	</wpt>

	A form element is <dfn id="required-pseudo">:required</dfn> or
	<dfn id="optional-pseudo">:optional</dfn>
	if a value for it is, respectively, required or optional before the
	form it belongs to can be validly submitted. Elements that are not
	form elements are neither required nor optional.

<h4 id="user-pseudos">
The User-interaction Pseudo-classes: '':user-valid'' and '':user-invalid''</h4>

	<wpt>
		invalidation/user-valid-user-invalid.html
		user-invalid-form-submission-invalidation.html
		user-invalid.html
		user-valid.html
		valid-invalid-form-fieldset.html
	</wpt>

	The <dfn id="user-invalid-pseudo">:user-invalid</dfn>
	and the <dfn id="user-valid-pseudo">:user-valid</dfn> pseudo-classes
	represent an element with incorrect or correct input, respectively,
	but only <em>after</em> the user has significantly interacted with it.
	Their purpose is to help the user identify mistakes in their input.

	These pseudo-classes must at least match their respective elements
	after the user has attempted to submit the form
	and before the user has interacted again with the input element
	or reset the form.
	They may also match at other times,
	as would be appropriate for highlighting an error to the user.
	For example, '':user-invalid''
	could start matching an '':invalid'' input element
	once the user has changed its value
	and focus has moved to another element;
	or stop matching only after
	the user has successfully corrected the input.

	Host languages may define more precise matching rules
	or defer to platform conventions;
	otherwise the exact behavior is UA-defined.
	Regardless,
	the '':user-invalid'' pseudo-class must only match '':invalid'' elements;
	and the '':user-valid'' pseudo-class must only match '':valid'' elements.
	See the <a spec=html lt=":user-invalid" selector>HTML specification</a>
	for the specific rules pertaining to HTML elements. [[!HTML]]

	<div class='example'>
		For example, the <{input}> element in the following document fragment
		would match '':invalid'' as soon as the page is loaded
		(because the <{input}>'s initial value violates its max constraint)
		but it won't match '':user-invalid''
		until the user significantly interacts with the input field
		or attempts to submit the form it's part of.

		<pre>
		&lt;form>
		  &lt;label>
		    Volume:
		    &lt;input name='vol' type=number min=0 max=10 value=11>
		  &lt;/label>
		  ...
		&lt;/form>
		</pre>
	</div>


<h2 id="structural-pseudos">
Tree-Structural pseudo-classes</h2>

	<wpt>
		selector-structural-pseudo-root.html
	</wpt>

	Selectors introduces the concept of <dfn export id="structural-pseudo-classes">structural pseudo-classes</dfn>
	to permit selection based on extra information that lies in
	the document tree but cannot be represented by other simple selectors or
	combinators.

	Standalone text and other non-element
	nodes are not counted when calculating the position of an element in the list
	of children of its parent. When calculating the position of an element in
	the list of children of its parent, the index numbering starts at 1.

	The <a>structural pseudo-classes</a> only apply to elements in the document tree;
	they must never match <a>pseudo-elements</a>.


<h3 id="the-root-pseudo">
'':root'' pseudo-class</h3>

	<wpt>
		root-siblings.html
	</wpt>

	The <dfn id='root-pseudo'>:root</dfn> pseudo-class represents an element that is
	the root of the document.

	For example, in a DOM document,
	the '':root'' pseudo-class matches the [=document element=].
	In HTML, this will be the <{html}> element
	(unless scripting has been used to modify the document).


<h3 id="the-empty-pseudo">
'':empty'' pseudo-class</h3>

	<wpt>
		selectors-empty-001.xml
	</wpt>

	The <dfn id='empty-pseudo'>:empty</dfn> pseudo-class represents
	an element that has no children
	except, optionally, <a>document white space characters</a>.
	In terms of the document tree,
	only element nodes and content nodes
	(such as [[DOM]] text nodes, and entity references)
	whose data has a non-zero length must be considered as affecting emptiness;
	comments, processing instructions, and other nodes
	must not affect whether an element is considered empty or not.

	<div class="example">
		Examples:
		''p:empty'' is a valid representation of the <{p}> elements
		in the following HTML fragment:

		<pre class="html">
			&lt;p>&lt;/p>
			&lt;p>
			&lt;p> &lt;/p>
			&lt;p> &lt;!-- comment -->&lt;/p>
		</pre>

		''div:empty'' is not a valid representation of the <code>&lt;div></code> elements
		in the following fragment:

		<pre class="html">
			&lt;div>text&lt;/div>
			&lt;div>&lt;p>&lt;/p>&lt;/div>
			&lt;div>&amp;nbsp;&lt;/div>
			&lt;div>&lt;p>bla&lt;/p>&lt;/div>
			&lt;div>this is not &lt;p>:empty&lt;/p>&lt;/div>
		</pre>
	</div>

	Note: In Level 2 and Level 3 of Selectors,
	'':empty'' did not match elements that contained only white space.
	This was changed so that that--
	given white space is largely collapsible in HTML
	and is therefore used for source code formatting,
	and especially because elements with omitted end tags
	are likely to absorb such white space into their DOM text contents--
	elements which authors perceive of as empty
	can be selected by this selector, as they expect.


<h3 id='child-index'>
Child-indexed Pseudo-classes</h3>

	<wpt>
		child-indexed-no-parent.html
		child-indexed-pseudo-class.html
		invalidation/negated-nth-child-when-ancestor-changes.html
		invalidation/negated-nth-last-child-when-ancestor-changes.html
		invalidation/nth-child-containing-ancestor.html
		invalidation/nth-child-in-shadow-root.html
		invalidation/nth-child-of-attr-largedom.html
		invalidation/nth-child-of-attr.html
		invalidation/nth-child-of-class-prefix.html
		invalidation/nth-child-of-class.html
		invalidation/nth-child-of-has.html
		invalidation/nth-child-of-id-prefix.html
		invalidation/nth-child-of-ids.html
		invalidation/nth-child-of-in-ancestor.html
		invalidation/nth-child-of-in-is.html
		invalidation/nth-child-of-in-shadow-root.html
		invalidation/nth-child-of-is.html
		invalidation/nth-child-of-pseudo-class.html
		invalidation/nth-child-of-sibling.html
		invalidation/nth-child-when-ancestor-changes.html
		invalidation/nth-child-when-sibling-changes.html
		invalidation/nth-child-whole-subtree.html
		invalidation/nth-last-child-containing-ancestor.html
		invalidation/nth-last-child-in-shadow-root.html
		invalidation/nth-last-child-of-attr.html
		invalidation/nth-last-child-of-class-prefix.html
		invalidation/nth-last-child-of-class.html
		invalidation/nth-last-child-of-has.html
		invalidation/nth-last-child-of-id-prefix.html
		invalidation/nth-last-child-of-ids.html
		invalidation/nth-last-child-of-in-ancestor.html
		invalidation/nth-last-child-of-in-is.html
		invalidation/nth-last-child-of-in-shadow-root.html
		invalidation/nth-last-child-of-is.html
		invalidation/nth-last-child-of-pseudo-class.html
		invalidation/nth-last-child-of-sibling.html
		invalidation/nth-last-child-when-ancestor-changes.html
		invalidation/nth-last-child-when-sibling-changes.html
		nth-child-and-nth-last-child.html
		nth-child-of-attribute.html
		nth-child-of-classname-002.html
		nth-child-of-classname.html
		nth-child-of-complex-selector-many-children-2.html
		nth-child-of-complex-selector-many-children.html
		nth-child-of-complex-selector.html
		nth-child-of-compound-selector.html
		nth-child-of-has.html
		nth-child-of-nesting.html
		nth-child-of-no-space-after-of.html
		nth-child-of-not.html
		nth-child-of-nth-child.html
		nth-child-of-pseudo.html
		nth-child-of-tagname.html
		nth-child-of-universal-selector.html
		nth-child-specificity-1.html
		nth-child-specificity-2.html
		nth-child-specificity-3.html
		nth-child-specificity-4.html
		nth-child-spurious-brace-crash.html
		nth-last-child-invalid.html
		nth-last-child-of-classname.html
		nth-last-child-of-complex-selector.html
		nth-last-child-of-compound-selector.html
		nth-last-child-of-nesting.html
		nth-last-child-of-no-space-after-of.html
		nth-last-child-of-style-sharing-1.html
		nth-last-child-of-style-sharing-2.html
		nth-last-child-of-tagname.html
		nth-last-child-specificity-1.html
		nth-last-child-specificity-2.html
		nth-last-child-specificity-3.html
		nth-last-child-specificity-4.html
		sharing-in-svg-use.html
		parsing/parse-anplusb.html
	</wpt>

	The pseudo-classes defined in this section select elements
	based on their index amongst their <a>inclusive siblings</a>.

	Note: Selectors 3 described these selectors as selecting elements based on their index in the child list of their parents.
	(This description survives in the name of this very section, and the names of several of the pseudo-classes.)
	As there was no reason to exclude them from matching elements without parents,
	or with non-element parents,
	they have been rephrased to refer to an element's relative index amongst its siblings.

<h4 id="the-nth-child-pseudo">
'':nth-child()'' pseudo-class</h4>

	<wpt>
		nth-of-invalid.html
		invalidation/crashtests/nth-child-of-attribute-crash.html
		invalidation/nth-of-namespace-class-invalidation-crash.html
	</wpt>

	The <dfn id='nth-child-pseudo' lt=":nth-child()">:nth-child(<var>An+B</var> [of <var>S</var>]? )</dfn>
	pseudo-class notation represents elements that
	are among <var>An+B</var>th elements
	from the list composed of
	their <a>inclusive siblings</a> that match the <a>selector list</a> <var>S</var>,
	which is a <<complex-real-selector-list>>.
	If <var>S</var> is omitted,
	it defaults to ''*|*''.

	The <var>An+B</var> notation and its interpretation
	are defined in [[css-syntax-3#anb-microsyntax]];
	it represents any index <var>i</var> = <var>A</var><var>n</var> + <var>B</var>
	for any non-negative integer <var>n</var>.

	Note: For these purposes, the list of elements is <b>1-indexed</b>;
	that is, the first child of an element has index 1, and will be matched by '':nth-child(2n+1)'',
	because when <code>n=0</code> the expression evaluates to ''1''.

	For example, this selector could address every other row in a table,
	and could be used to alternate the color of paragraph text in a cycle of four.

	<div class="example">
		Examples:

		<pre>
		:nth-child(even)   /* represents the 2nd, 4th, 6th, etc elements
		:nth-child(10n-1)  /* represents the 9th, 19th, 29th, etc elements */
		:nth-child(10n+9)  /* Same */
		:nth-child(10n+-1) /* Syntactically invalid, and would be ignored */
		</pre>
	</div>

	Note: The specificity of the '':nth-child()'' pseudo-class is
	the specificity of a single pseudo-class plus,
	if <var>S</var> is specified,
	the specificity of the most specific <a>complex selector</a> in <var>S</var>.
	See [[#specificity-rules]].
	Thus ''<var>S</var>:nth-child(<var>An+B</var>)''
	and '':nth-child(<var>An+B</var> of <var>S</var>)''
	have the exact same specificity,
	although they do differ in behavior
	(see example below).

	<div class="example">
		By passing a selector argument,
		we can select the Nth element that matches that selector.
		For example, the following selector matches the first three “important” list items,
		denoted by the ''.important'' class:

		<pre>:nth-child(-n+3 of li.important)</pre>

		Note that this is different from moving the selector outside of the function, like:

		<pre>li.important:nth-child(-n+3)</pre>

		This selector instead just selects the first three children
		if they also happen to be "important" list items.
	</div>

	<div class="example">
		Here's another example of using the selector argument,
		to ensure that zebra-striping a table works correctly.

		Normally, to zebra-stripe a table's rows,
		an author would use CSS similar to the following:

		<pre>
		tr {
			background: white;
		}
		tr:nth-child(even) {
			background: silver;
		}
		</pre>

		However, if some of the rows are hidden and not displayed,
		this can break up the pattern,
		causing multiple adjacent rows to have the same background color.
		Assuming that rows are hidden with the ''[hidden]'' attribute in HTML,
		the following CSS would zebra-stripe the table rows robustly,
		maintaining a proper alternating background
		regardless of which rows are hidden:

		<pre>
		tr {
			background: white;
		}
		tr:nth-child(even of :not([hidden])) {
			background: silver;
		}
		</pre>
	</div>

<h4 id="the-nth-last-child-pseudo">
'':nth-last-child()'' pseudo-class</h4>

	The <dfn id='nth-last-child-pseudo' lt=":nth-last-child()">:nth-last-child(<var>An+B</var> [of <var>S</var>]? )</dfn>
	pseudo-class notation represents elements that
	are among <var>An+B</var>th elements
	from the list composed of
	their <a>inclusive siblings</a> that match the <a>selector list</a> <var>S</var>,
	counting backwards from the end.
	<var>S</var> is <<complex-real-selector-list>>.
	If <var>S</var> is omitted,
	it defaults to ''*|*''.

	Note: The specificity of the '':nth-last-child()'' pseudo-class,
	like the '':nth-child()'' pseudo-class,
	combines the specificity of a regular pseudo-class
	with that of its selector argument <var>S</var>.
	See [[#specificity-rules]].

	The CSS Syntax Module [[!CSS3SYN]] defines the <a href="https://un5n798jx6qx6j0rmf2verhh.julianrbryant.com/css-syntax/#anb"><var>An+B</var> notation</a>.

	<div class="example">
		Examples:

		<pre>
		tr:nth-last-child(-n+2)    /* represents the two last rows of an HTML table */

		foo:nth-last-child(odd)    /* represents all odd foo elements in their parent element,
		                              counting from the last one */
		</pre>
	</div>

<h4 id="the-first-child-pseudo">
'':first-child'' pseudo-class</h4>

	<wpt>
		first-child.html
		invalidation/first-child-last-child.html
	</wpt>

	The <dfn id='first-child-pseudo'>:first-child</dfn> pseudo-class
	represents an element that is first among its <a>inclusive siblings</a>.
	Same as '':nth-child(1)''.

	<div class="example">
		Examples:
		The following selector represents a <a element>p</a> element that is
		the first child of a <a element>div</a> element:

		<pre>div > p:first-child</pre>

		This selector can represent the <code>p</code> inside the
		<code>div</code> of the following fragment:

		<pre>
		&lt;p> The last P before the note.&lt;/p>
		&lt;div class="note">
			 &lt;p> The first P inside the note.&lt;/p>
		&lt;/div>
		</pre>

		but cannot represent the second <code>p</code> in the following fragment:

		<pre>
		&lt;p> The last P before the note.&lt;/p>
		&lt;div class="note">
			 &lt;h2> Note &lt;/h2>
			 &lt;p> The first P inside the note.&lt;/p>
		&lt;/div>
		</pre>

		The following two selectors are usually equivalent:

		<pre>
		* > a:first-child /* a is first child of any element */
		a:first-child /* Same (assuming a is not the root element) */
		</pre>
	</div>

<h4 id="the-last-child-pseudo">
'':last-child'' pseudo-class</h4>

	<wpt>
		last-child.html
	</wpt>

	The <dfn id='last-child-pseudo'>:last-child</dfn> pseudo-class
	represents an element that is last among its <a>inclusive siblings</a>.
	Same as '':nth-last-child(1)''.

	<div class="example">
		Example:
		The following selector represents a list item <code>li</code> that
		is the last child of an ordered list <code>ol</code>.

		<pre>ol > li:last-child</pre>
	</div>

<h4 id="the-only-child-pseudo">
'':only-child'' pseudo-class</h4>

	<wpt>
		only-child.html
	</wpt>

	The <dfn id='only-child-pseudo'>:only-child</dfn> pseudo-class
	represents an element that has no siblings.
	Same as '':first-child:last-child''
	or '':nth-child(1):nth-last-child(1)'',
	but with a lower specificity.


<h3 id='typed-child-index'>
Typed Child-indexed Pseudo-classes</h3>

	<wpt>
		nth-of-type-namespace.html
	</wpt>

	The pseudo-classes in this section are similar to the <a href="#child-index">Child Index Pseudo-classes</a>,
	but they resolve based on an element's index <strong>among elements of the same <a href="#type-selectors">type (tag name)</a></strong> in their sibling list.

<h4 id="the-nth-of-type-pseudo">
'':nth-of-type()'' pseudo-class</h4>

	The <dfn id='nth-of-type-pseudo' lt=":nth-of-type()">:nth-of-type(<var>An+B</var>)</dfn> pseudo-class notation
	represents the same elements that would be matched by '':nth-child(|An+B| of |S|)'',
	where |S| is a [=type selector=] and namespace prefix matching the element in question.
	For example,
	when considering whether an HTML <{img}> element matches this [=pseudo-class=],
	the |S| in question is ''html|img''
	(assuming an appropriate <code>html</code> namespace is declared).

	<div class="example">
		CSS example:
		This allows an author to alternate the position of floated images:

		<pre>
		img:nth-of-type(2n+1) { float: right; }
		img:nth-of-type(2n) { float: left; }
		</pre>
	</div>

	Note: If the type of the element is known ahead of time,
	this pseudo-class is equivalent to using '':nth-child()'' with a type selector.
	That is, ''img:nth-of-type(2)''
	is equivalent to ''*:nth-child(2 of img)''.

<h4 id="the-nth-last-of-type-pseudo">
'':nth-last-of-type()'' pseudo-class</h4>

	<wpt>
		last-of-type.html
	</wpt>

	The <dfn id='nth-last-of-type-pseudo' lt=":nth-last-of-type()">:nth-last-of-type(<var>An+B</var>)</dfn> pseudo-class notation
	represents the same elements that would be matched by '':nth-last-child(|An+B| of |S|)'',
	where |S| is a [=type selector=] and namespace prefix matching the element in question.
	For example,
	when considering whether an HTML <{img}> element matches this [=pseudo-class=],
	the |S| in question is ''html|img''
	(assuming an appropriate <code>html</code> namespace is declared).

	<div class="example">
		Example:
		To represent all <code>h2</code> children of an XHTML
		<code>body</code> except the first and last, one could use the
		following selector:

		<pre>body > h2:nth-of-type(n+2):nth-last-of-type(n+2) </pre>

		In this case, one could also use '':not()'', although the
		selector ends up being just as long:

		<pre>body > h2:not(:first-of-type):not(:last-of-type) </pre>
	</div>

<h4 id="the-first-of-type-pseudo">
'':first-of-type'' pseudo-class</h4>

	<wpt>
		first-of-type.html
		invalidation/negated-always-matches-negated-first-of-type-when-ancestor-changes.html
		invalidation/negated-first-of-type-in-nonsubject-position.html
		invalidation/negated-is-always-matches-negated-first-of-type-when-ancestor-changes.html
		invalidation/negated-is-never-matches-negated-first-of-type-when-ancestor-changes.html
		invalidation/negated-negated-first-of-type-when-ancestor-changes.html
		invalidation/negated-never-matches-negated-first-of-type-when-ancestor-changes.html
		of-type-selectors.xhtml
	</wpt>

	The <dfn id='first-of-type-pseudo'>:first-of-type</dfn> pseudo-class
	represents the same element as '':nth-of-type(1)''.

	<div class="example">
		Example:
		The following selector represents a definition title
		<code>dt</code> inside a definition list <code>dl</code>, this
		<code>dt</code> being the first of its type in the list of children of
		its parent element.

		<pre>dl dt:first-of-type</pre>

		It is a valid description for the first two <code>dt</code>
		elements in the following example but not for the third one:

		<pre>
		&lt;dl>
		  &lt;dt>gigogne&lt;/dt>
		  &lt;dd>
		    &lt;dl>
		      &lt;dt>fus&eacute;e&lt;/dt>
		      &lt;dd>multistage rocket&lt;/dd>
		      &lt;dt>table&lt;/dt>
		      &lt;dd>nest of tables&lt;/dd>
		    &lt;/dl>
		  &lt;/dd>
		&lt;/dl>
		</pre>
	</div>

<h4 id="the-last-of-type-pseudo">
'':last-of-type'' pseudo-class</h4>

	<wpt>
		invalidation/negated-always-matches-negated-last-of-type-when-ancestor-changes.html
		invalidation/negated-is-always-matches-negated-last-of-type-when-ancestor-changes.html
		invalidation/negated-is-never-matches-negated-last-of-type-when-ancestor-changes.html
		invalidation/negated-last-of-type-invalidation.html
		invalidation/negated-negated-last-of-type-when-ancestor-changes.html
		invalidation/negated-never-matches-negated-last-of-type-when-ancestor-changes.html
	</wpt>

	The <dfn id='last-of-type-pseudo'>:last-of-type</dfn> pseudo-class
	represents the same element as '':nth-last-of-type(1)''.

	<div class="example">
		Example:
		The following selector represents the last data cell
		<code>td</code> of a table row <code>tr</code>.

		<pre>tr > td:last-of-type</pre>
	</div>

<h4 id="the-only-of-type-pseudo">
'':only-of-type'' pseudo-class</h4>

	<wpt>
		only-of-type.html
	</wpt>

	The <dfn id='only-of-type-pseudo'>:only-of-type</dfn> pseudo-class
	represents the same element as '':first-of-type:last-of-type''.


<h2 id="combinators">
Combinators</h2>

<h3 id="descendant-combinators">
Descendant combinator (<code> </code>)</h3>

	<wpt>
		parsing/parse-descendant.html
	</wpt>

	At times, authors may want selectors to describe an element that is
	the descendant of another element in the document tree (e.g., "an
	<a element>em</a> element that is contained within an <a element>H1</a> element").
	The <dfn export>descendant combinator</dfn> expresses such a relationship.

	A descendant combinator is whitespace that separates two <a>compound selectors</a>.

	A selector of the form ''A B'' represents an element <code>B</code> that is an
	arbitrary descendant of some ancestor element <code>A</code>.

	<div class="example">
		Examples:
		For example, consider the following selector:

		<pre>h1 em</pre>

		It represents an <a element>em</a> element being the descendant of
		an <a element>h1</a> element. It is a correct and valid, but partial,
		description of the following fragment:

		<pre>
		&lt;h1>This &lt;span class="myclass">headline
		is &lt;em>very&lt;/em> important&lt;/span>&lt;/h1>
		</pre>

		The following selector:

		<pre>div * p</pre>

		represents a <a element>p</a> element that is a grandchild or later
		descendant of a <a element>div</a> element. Note the whitespace on
		either side of the "*" is not part of the universal selector; the
		whitespace is a combinator indicating that the <code>div</code> must be the
		ancestor of some element, and that that element must be an ancestor
		of the <code>p</code>.
		The following selector, which combines descendant combinators and
		<a href="#attribute-selectors">attribute selectors</a>, represents an
		element that (1) has the <code>href</code> attribute set and (2) is
		inside a <code>p</code> that is itself inside a <code>div</code>:

		<pre>div p *[href]</pre>
	</div>


<h3 id="child-combinators">
Child combinator (<code>></code>)</h3>

	<wpt>
		parsing/parse-child.html
	</wpt>

	A <dfn export>child combinator</dfn> describes a childhood relationship
	between two elements. A child combinator is made of the
	&quot;greater-than sign&quot; (U+003E, <dfn selector id=selectordef-child>></dfn>) code point and
	separates two <a>compound selectors</a>.

	<div class="example">
		Examples:
		The following selector represents a <a element>p</a> element that is
		child of <code>body</code>:

		<pre>body > p</pre>

		The following example combines descendant combinators and child
		combinators.

		<pre>div ol>li p</pre>
		<!-- LEAVE THOSE SPACES OUT! see below -->

		It represents a <a element>p</a> element that is a descendant of an
		<a element>li</a> element; the <a element>li</a> element must be the
		child of an <a element>ol</a> element; the <a element>ol</a> element must
		be a descendant of a <code>div</code>. Notice that the optional white
		space around the ">" combinator has been left out.
	</div>

	For information on selecting the first child of an element,
	please see the section on the '':first-child'' pseudo-class above.


<h3 id="adjacent-sibling-combinators">
Next-sibling combinator (<code>+</code>)</h3>

	<wpt>
		invalidation/insert-sibling-001.html
		invalidation/insert-sibling-002.html
		invalidation/insert-sibling-003.html
		invalidation/insert-sibling-004.html
		invalidation/sibling.html
		parsing/parse-sibling.html
	</wpt>

	The <dfn export id="next-sibling-combinator">next-sibling combinator</dfn> is made of the “plus sign”
	(U+002B, <dfn selector id=selectordef-adjacent>+</dfn>) code point that separates two
	<a>compound selectors</a>.
	The elements represented by the two <a>compound selectors</a>
	share the same parent in the document tree
	and the element represented by the first <a>compound selector</a>
	immediately precedes the element represented by the second one.
	Non-element nodes (e.g. text between elements)
	are ignored when considering the adjacency of elements.

	<div class="example">
		Examples:
		The following selector represents a <a element>p</a> element
		immediately following a <a element>math</a> element:

		<pre>math + p</pre>

		The following selector is conceptually similar to the one in the
		previous example, except that it adds an attribute selector &mdash; it
		adds a constraint to the <a element>h1</a> element, that it must have
		<code>class="opener"</code>:

		<pre>h1.opener + h2</pre>
	</div>


<h3 id="general-sibling-combinators">
Subsequent-sibling combinator (<code>~</code>)</h3>

	The <dfn export id="subsequent-sibling-combinator">subsequent-sibling combinator</dfn> is made of the &quot;tilde&quot;
	(U+007E, <dfn selector id=selectordef-sibling>~</dfn>) code point that separates two <a>compound selectors</a>.
	The elements represented by the two <a>compound selectors</a> share
	the same parent in the document tree and the element represented by
	the first compound selector precedes (not necessarily immediately) the element
	represented by the second one.

	<div class="example">
		<pre>h1 ~ pre</pre>

		represents a <a element>pre</a> element following an <code>h1</code>. It
		is a correct and valid, but partial, description of:

		<pre>
		&lt;h1>Definition of the function a&lt;/h1>
		&lt;p>Function a(x) has to be applied to all figures in the table.&lt;/p>
		&lt;pre>function a(x) = 12x/13.5&lt;/pre>
		</pre>
	</div>


<h2 id="specificity-rules">
Calculating a selector's specificity</h2>

	A selector's <dfn export>specificity</dfn> is calculated for a given element as follows:

	<ul>
		<li>count the number of ID selectors in the selector (= <var>A</var>)
		<li>count the number of class selectors, attributes selectors, and pseudo-classes in the selector (= <var>B</var>)
		<li>count the number of type selectors and pseudo-elements in the selector (= <var>C</var>)
		<li>ignore the universal selector
	</ul>

	If the selector is a <a>selector list</a>,
	this number is calculated for each selector in the list.
	For a given matching process against the list, the specificity in effect
	is that of the most specific selector in the list that matches.

	A few pseudo-classes provide “evaluation contexts” for other selectors,
	and so have their specificity defined specially:

	<ul>
		<li>
			The specificity of an '':is()'', '':not()'', or '':has()'' pseudo-class
			is replaced by the specificity of
			the most specific <a>complex selector</a>
			in its <a>selector list</a> argument.
		<li>
			Analogously, the specificity of an '':nth-child()'' or '':nth-last-child()'' selector
			is the specificity of the pseudo-class itself (counting as one pseudo-class selector)
			plus the specificity of
			the most specific <a>complex selector</a>
			in its <a>selector list</a> argument (if any).
		<li>
			The specificity of a '':where()'' pseudo-class is replaced by zero.
	</ul>

	<div class="example">
		For example:
		<ul>
			<li>
				'':is(em, #foo)'' has
				a specificity of (1,0,0)--
				like an ID selector (''#foo'')--
				when matched against any of
				<code>&lt;em></code>, <code>&lt;p id=foo></code>, or <code>&lt;em id=foo></code>.
			<li>
				''.qux:where(em, #foo#bar#baz)'' has
				a specificity of (0,1,0):
				only the ''.qux'' outside the '':where()''
				contributes to selector specificity.
			<li>
				'':nth-child(even of li, .item)'' has
				a specificity of (0,2,0)--
				like a class selector (''.item'') plus a pseudo-class--
				when matched against any of
<code>&lt;li></code>, <code>&lt;ul class=item></code>, or <code>&lt;li class=item id=foo></code>.
			<li>
				'':not(em, strong#foo)'' has
				a specificity of (1,0,1)--
				like a tag selector (<css>strong</css>) combined with an ID selector (''#foo'')--
				when matched against any element.
		</ul>
	</div>

	Specificities are compared by comparing the three components in order:
	the specificity with a larger <var>A</var> value is more specific;
	if the two <var>A</var> values are tied,
	then the specificity with a larger <var>B</var> value is more specific;
	if the two <var>B</var> values are also tied,
	then the specificity with a larger <var>C</var> value is more specific;
	if all the values are tied,
	the two specificities are equal.

	Due to storage limitations,
	implementations may have limitations on the size of <var>A</var>, <var>B</var>, or <var>C</var>.
	If so, values higher than the limit must be clamped to that limit,
	and not overflow.

	<div class="example">
		Examples:
		<pre>
		*               /* a=0 b=0 c=0 */
		LI              /* a=0 b=0 c=1 */
		UL LI           /* a=0 b=0 c=2 */
		UL OL+LI        /* a=0 b=0 c=3 */
		H1 + *[REL=up]  /* a=0 b=1 c=1 */
		UL OL LI.red    /* a=0 b=1 c=3 */
		LI.red.level    /* a=0 b=2 c=1 */
		#x34y           /* a=1 b=0 c=0 */
		#s12:not(FOO)   /* a=1 b=0 c=1 */
		.foo :is(.bar, #baz)
		                /* a=1 b=1 c=0 */
		</pre>
	</div>

	Note: Repeated occurrences of the
	same simple selector are allowed and do increase specificity.

	Note: The specificity of the styles
	specified in an HTML <code>style</code> attribute
	<a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/css-style-attr/#interpret">is described in CSS Style Attributes</a>. [[CSSSTYLEATTR]]


<h2 id="grammar" oldids="formal-syntax">
Grammar</h2>

	<wpt>
		selectors-attr-white-space-001.html
	</wpt>

	Selectors are [=CSS/parsed=] according to the following grammar:

	<pre class=prod>
	<dfn>&lt;selector-list></dfn> = <<complex-selector-list>>

	<dfn>&lt;complex-selector-list></dfn> = <<complex-selector>>#
	<dfn>&lt;complex-real-selector-list></dfn> = <<complex-real-selector>>#

	<dfn>&lt;compound-selector-list></dfn> = <<compound-selector>>#

	<dfn>&lt;simple-selector-list></dfn> = <<simple-selector>>#

	<dfn>&lt;relative-selector-list></dfn> = <<relative-selector>>#
	<dfn>&lt;relative-real-selector-list></dfn> = <<relative-real-selector>>#


	<dfn>&lt;complex-selector></dfn> = <<complex-selector-unit>> [ <<combinator>>? <<complex-selector-unit>> ]*
	<dfn>&lt;complex-selector-unit></dfn> = [ <<compound-selector>>? <<pseudo-compound-selector>>* ]!
	<dfn>&lt;complex-real-selector></dfn> = <<compound-selector>> [ <<combinator>>? <<compound-selector>> ]*

	<dfn>&lt;relative-selector></dfn> = <<combinator>>? <<complex-selector>>
	<dfn>&lt;relative-real-selector></dfn> = <<combinator>>? <<complex-real-selector>>

	<dfn>&lt;compound-selector></dfn> = [ <<type-selector>>? <<subclass-selector>>* ]!
	<dfn>&lt;pseudo-compound-selector></dfn> =  <<pseudo-element-selector>> <<pseudo-class-selector>>*

	<dfn>&lt;simple-selector></dfn> = <<type-selector>> | <<subclass-selector>>


	<dfn>&lt;combinator></dfn> = '>' | '+' | '~' | [ '|' '|' ]

	<dfn noexport>&lt;wq-name></dfn> = <<ns-prefix>>? <<ident-token>>
	<dfn noexport>&lt;ns-prefix></dfn> = [ <<ident-token>> | '*' ]? '|'

	<dfn>&lt;type-selector></dfn> = <<wq-name>> | <<ns-prefix>>? '*'

	<dfn noexport>&lt;subclass-selector></dfn> = <<id-selector>> | <<class-selector>> |
	                      <<attribute-selector>> | <<pseudo-class-selector>>

	<dfn>&lt;id-selector></dfn> = <<hash-token>>

	<dfn>&lt;class-selector></dfn> = '.' <<ident-token>>

	<dfn>&lt;attribute-selector></dfn> = '[' <<wq-name>> ']' |
	    '[' <<wq-name>> <<attr-matcher>> [ <<string-token>> | <<ident-token>> ] <<attr-modifier>>? ']'
	<dfn noexport>&lt;attr-matcher></dfn> = [ '~' | '|' | '^' | '$' | '*' ]? '='
	<dfn noexport>&lt;attr-modifier></dfn> = i | s

	<dfn>&lt;pseudo-class-selector></dfn> = : <<ident-token>> |
	                          : <<function-token>> <<any-value>> )

	<dfn>&lt;pseudo-element-selector></dfn> = : <<pseudo-class-selector>> | <<legacy-pseudo-element-selector>>
	<dfn>&lt;legacy-pseudo-element-selector></dfn> =  : [before | after | first-line | first-letter]
	</pre>

	In interpreting the above grammar,
	the following rules apply:

	<ul>
	<li id="white-space">
		White space is forbidden:
		* Between any of the top-level components of a <<compound-selector>>
			or <<complex-selector-unit>>
			(that is, forbidden between the <<type-selector>> and <<subclass-selector>>,
			or between the <<pseudo-element-selector>> and <<pseudo-class-selector>>,
			etc).
		* Between <em>any</em> of the components of a <<type-selector>>
			or a <<class-selector>>.
		* Between the ':'s, or between the ':' and <<ident-token>> or <<function-token>>,
			of a <<pseudo-element-selector>> or a <<pseudo-class-selector>>.
		* Between <em>any</em> of the components of a <<wq-name>>.
		* Between the components of an <<attr-matcher>>.
		* Between the <<compound-selector>> or <<pseudo-compound-selector>>s
			in a <<complex-selector-unit>>
		* Between the components of a <<combinator>>.

		Whitespace is <em>required</em>
		between two <<complex-selector-unit>>s
		if the <<combinator>> between them is omitted.
		(This indicates the descendant combinator is being used.)
	<li>
		In <<id-selector>>, the <<hash-token>>’s value must be an <a>identifier</a>.
	<li>
		The <<pseudo-class-selector>> production
		excludes the <<legacy-pseudo-element-selector>> production.
		(That is, '':before''/etc must never be parsed as a pseudo-class,
		even if doing so would cause the selector to become valid
		due to, for example, other simple selectors following it.)
	</ul>

	Note: A selector is also subject to a variety of more specific syntactic constraints,
	and adherence to the grammar above is necessary <em>but not sufficient</em>
	for the selector to be considered valid.
	See [[#invalid]] for additional rules for parsing selectors.

	Note: In general,
	a <<pseudo-element-selector>> is only valid
	if placed at the end of the last <<compound-selector>>
	in a <<complex-selector>>.
	In some circumstances, however,
	it can be followed by more <<pseudo-element-selector>>s or <<pseudo-class-selector>>s;
	but these are specified on a case-by-case basis.
	(For example, the <a>user action pseudo-classes</a> are allowed after any <a>pseudo-element</a>,
	and the <a>tree-abiding pseudo-elements</a>
	are allowed after the ''::slotted()'' pseudo-element.)

	<p class=note id=single-colon-pseudos>
		The four <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/CSS2/selectors.html#pseudo-element-selectors">Level 2</a> <a>pseudo-elements</a>
		(''::before'', ''::after'', ''::first-line'', and ''::first-letter'')
		may, for legacy reasons,
		be written with only a single ":" character at their front,
		making them resemble a <<pseudo-class-selector>>.


<h3 id='forgiving-selector'>
<<forgiving-selector-list>></h3>

	For legacy reasons,
	the general behavior of a selector list
	is that if any selector in the list fails to parse
	(because it uses new or UA-specific selector features, for instance),
	the entire selector list becomes invalid.
	This can make it hard to write CSS that uses new selectors
	and still works correctly in older user agents.

	The <dfn><<forgiving-selector-list>></dfn> production
	instead parses each selector in the list individually,
	simply ignoring ones that fail to parse,
	so the remaining selectors can still be used.

	Note: Style rules still use the normal, unforgiving selector list behavior.
	<<forgiving-selector-list>> is used in '':is()'' and '':where()'' only.
	Although it does have some minor implications on specificity,
	wrapping a style rule's selector in '':is()''
	effectively "upgrades" it to become forgiving,
	so long as it doesn't contain any pseudo-elements
	(which aren't valid in '':is()'' or '':where()'').

	Syntactically, <<forgiving-selector-list>> is equivalent to <code><<any-value>>?</code>.
	It is then [=parsed as a forgiving selector list=]
	to obtain its actual value.

	<div algorithm>
		To <dfn export>parse as a forgiving selector list</dfn> given an input |input|:

		1. [=Parse a list=] of <<complex-real-selector>>s from |input|,
			and let |selector list| be the result.

		3. Remove all failure items from |selector list|,
			and all items that are [=invalid selectors=],
			then return a <<selector-list>>
			representing the remaining items in |selector list|.
			(This might be empty.)
	</div>

	Any items in a <<forgiving-selector-list>> that are invalid
	(whether explicitly, by using unknown selectors or syntax,
	or merely contextually, using known syntax but in an invalid context)
	must be treated as having zero specificity.

	Note: <<forgiving-selector-list>> is intentionally used only in '':is()'' and '':where()'',
	not in any other selector that takes a selector argument.
	<!--
	This is both for bugwards compat with jQuery
	(https://github.com/w3c/csswg-drafts/issues/7676)
	and general ease of learning which selector arguments are allowed to be forgiving and which aren't.
	-->


<h2 id='api-hooks'>
API Hooks</h2>

	To aid in the writing of specs that use Selectors concepts,
	this section defines several API hooks that can be invoked by other specifications.

	Issue: Are these still necessary now that we have more rigorous definitions for <a>match</a> and <a>invalid selector</a>?
	Nouns are a lot easier to coordinate across specification than predicates,
	and details like the exact order of elements returned from <code>querySelector</code>
	seem to make more sense being defined in the DOM specification than in Selectors.


<h3 id='parse-selector' algorithm>
Parse A Selector</h3>

	This section defines how to <dfn export>parse a selector</dfn> from a string <var>source</var>.
	It returns either a complex selector list,
	or failure.

	<ol>
		<li>
			Let <var>selector</var> be the result of [=CSS/parsing=] <var>source</var> as a <<selector-list>>.
			If this returns failure,
			it's an [=invalid selector=];
			return failure.

		<li>
			If |selector| is an [=invalid selector=] for any other reason
			(such as, for example, containing an undeclared namespace prefix),
			return failure.

		<li>
			Otherwise,
			return <var>selector</var>.
	</ol>


<h3 id='parse-relative-selector' algorithm>
Parse A Relative Selector</h3>

	This section defines how to <dfn export>parse a relative selector</dfn> from a string <var>source</var>.
	It returns either a complex selector list,
	or failure.

	<ol>
		<li>
			Let <var>selector</var> be the result of [=CSS/parsing=] <var>source</var> as a <<relative-selector-list>>.
			If this returns failure,
			it's an [=invalid selector=];
			return failure.

		<li>
			If |selector| is an [=invalid selector=] for any other reason
			(such as, for example, containing an undeclared namespace prefix),
			return failure.

		<li>
			Otherwise,
			return <var>selector</var>.
	</ol>


<h3 id='match-against-element' algorithm>
Match a Selector Against an Element</h3>

	This section defines how to <dfn export>match a selector against an element</dfn>.

	APIs using this algorithm must provide a <var>selector</var> and an <var>element</var>.

	Callers may optionally provide:

	<ul>
		<li>
			one or more [=scoping roots=],
			for resolving the '':scope'' pseudo-class against.
	</ul>

	This algorithm returns either success or failure.

	For each <a>complex selector</a> in the given <var>selector</var>
	(which is taken to be a <a>list of complex selectors</a>),
	match the complex selector against <var>element</var>,
	as described in the following paragraph.
	If the matching returns success for any complex selector,
	then the algorithm return success; otherwise it returns failure.

	To <dfn>match a complex selector against an element</dfn>,
	process it <a>compound selector</a> at a time,
	in right-to-left order.
	This process is defined recursively as follows:

	<ul>
		<li>If any simple selectors in the rightmost compound selector
		does not match the element, return failure.

		<li>Otherwise, if there is only one compound selector in the
		complex selector, return success.

		<li>Otherwise, consider all possible elements
		that could be related to this element by the rightmost <a for=selector>combinator</a>.
		If the operation of matching the selector consisting of this selector
		with the rightmost compound selector and rightmost combinator removed
		against any one of these elements returns success, then return success.
		Otherwise, return failure.
	</ul>


<h3 id='match-against-pseudo-element' algorithm>
Match a Selector Against a Pseudo-element</h3>

	This section defines how to <dfn export>match a selector against a pseudo-element</dfn>.

	APIs using this algorithm must provide a <var>selector</var>
	and a <var>pseudo-element</var>.
	They may optionally provide the same things they may optionally provide
	to the algorithm to <a>match a selector against an element</a>.

	This algorithm returns success or failure.

	For each <a>complex selector</a> in the given <var>selector</var>, if both:
	<ul>
		<li>the rightmost <a>simple selector</a> in the complex selector
		matches <var>pseudo-element</var>, and

		<li>the result of running <a>match a complex selector against an element</a>
		on the remainder of the <a>complex selector</a>
		(with just the rightmost simple selector
		of its rightmost complex selector removed),
		<var>pseudo-element</var>'s corresponding element,
		and any optional parameters provided to this algorithm
		returns success,
	</ul>
	then return success.

	Otherwise
	(that is, if this doesn't happen for any of the complex selectors in <var>selector</var>),
	return failure.


<h3 id='match-against-tree' algorithm>
Match a Selector Against a Tree</h3>

	This section defines how to <dfn export>match a selector against a tree</dfn>.

	APIs using this algorithm must provide a selector,
	and one or more <var>root elements</var>
	indicating the [=tree|subtrees=] that will be searched by the selector.
	All of the <var>root elements</var> must share the same [=tree/root=],
	or else calling this algorithm is invalid.

	They may optionally provide:

	<ul>
		<li>
			One or more <a>scoping roots</a>
			indicating the selector is <a>scoped</a>.

		<li>
			A list of <a>pseudo-elements</a> that are allowed to show up in the match list.
			If not specified, this defaults to allowing all pseudo-elements.

			Issue: Only the [=tree-abiding pseudo-elements=] are really
			handled in any way remotely like this.
	</ul>

	This algorithm returns a (possibly empty) list of elements.

	<ol>
		<li>
			Start with a list of <var>candidate elements</var>,
			which are the <var>root elements</var>
			and all of their descendant elements,
			sorted in <a>shadow-including tree order</a>,
			unless otherwise specified.

		<li>
			If [=scoping root=] were provided,
			then remove from the <var>candidate elements</var>
			any elements that are not
			<a>descendants</a> of at least one <a>scoping root</a>.

		<li>
			Initialize the <var>selector match list</var> to empty.

		<li>
			For each <var>element</var> in the set of
			<var>candidate elements</var>:

			<ol>
				<li>
				If the result of <a>match a selector against an element</a>
				for <var>element</var> and <var>selector</var> is success,
				add <var>element</var> to the <var>selector match list</var>.

				<li>
				For each possible pseudo-element associated with <var>element</var>
				that is one of the pseudo-elements allowed to show up in the match list,
				if the result of <a>match a selector against a pseudo-element</a>
				for the pseudo-element and <var>selector</var> is success,
				add the pseudo-element to the <var>selector match list</var>.

				Issue: The relative position of pseudo-elements
				in <var>selector match list</var> is undefined.
				There's not yet a context that exposes this information,
				but we need to decide on something eventually,
				before something <em>is</em> exposed.
			</ol>

	</ol>


<h2 id='dom-mapping'>
Appendix A: Guidance on Mapping Source Documents &amp; Data to an Element Tree</h2>

	<em>This section is informative.</em>

	The element tree structure described by the DOM is powerful and useful,
	but generic enough to model pretty much any language that describes tree-based data
	(or even graph-based, with a suitable interpretation).

	Some languages, like HTML, already have well-defined procedures
	for producing a DOM object from a resource.
	If a given language does not,
	such a procedure must be defined
	in order for Selectors to apply to documents in that language.

	At minimum,
	the document language must define what maps to the DOM concept of an "element".

	The primary one-to-many relationship between nodes--
	parent/child in tree-based structures,
	element/neighbors in graph-based structures--
	should be reflected as the child nodes of an element.

	Other features of the element should be mapped
	to something that serves a similar purpose to the same feature in DOM:

	<dl>
		<dt>type
		<dd>
			If the elements in the document language have some notion of "type"
			as a basic distinguisher between different groups of elements,
			it should be reflected as the "type" feature.

			If this "type" can be separated into a "basic" name
			and a "namespace" that groups names into higher-level groups,
			the latter should be reflected as the "namespace" feature.
			Otherwise, the element shouldn't have a "namespace" feature,
			and the entire name should be reflected as the "type" feature.

		<dt>id
		<dd>
			If some aspect of the element functions as a unique identifier across the document,
			it should be mapped to the "id" feature.

			Note: While HTML only allows an element to have a single ID,
			this should not be taken as a general restriction.
			The important quality of an ID is that each ID should be associated with a single element;
			a single element can validly have multiple IDs.

		<dt>classes and attributes
		<dd>
			Aspects of the element that are useful for identifying the element,
			but are not generally unique to elements within a document,
			should be mapped to the "class" or "attribute" features
			depending on if they're something equivalent to a "label" (a string by itself)
			or a "property" (a name/value pair)

		<dt>pseudo-classes and pseudo-elements
		<dd>
			If any elements match any pseudo-classes or have any pseudo-elements,
			that must be explicitly defined.

			Issue: Some pseudo-classes are *syntactical*,
			like '':has()'' and '':is()'',
			and thus should always work.
			Need to indicate that somewhere.
			Probably the structural pseudos always work
			whenever the child list is ordered.
	</dl>

	<div class='example'>
		For example, <a href="https://github.com/lloyd/JSONSelect">JSONSelect</a> is a library that uses selectors
		to extract information from JSON documents.

		<ul>
			<li>
				The "elements" of the JSON document
				are each array, object, boolean, string, number, or null.
				The array and object elements have their contents as children.

			<li>
				Each element's type is its JS type name:
				"array", "object", etc.

			<li>
				Children of an object
				have their key as a class.

			<li>
				Children of an array match the '':first-child'', '':nth-child()'', etc pseudo-classes.

			<li>
				The root object matches '':root''.

			<li>
				It additionally defines '':val()'' and '':contains()'' pseudo-classes,
				for matching boolean/number/string elements with a particular value
				or which contain a particular substring.
		</ul>

		This structure is sufficient to allow powerful, compact querying of JSON documents with selectors.
	</div>


<h2 id="compat">
Appendix B: Obsolete but Required <code>-webkit-</code> Parsing Quirks for Web Compat</h2>

	<wpt>
		webkit-pseudo-element.html
	</wpt>

	<em>This appendix is normative.</em>

	Due to legacy Web-compat constraints,
	user agents expecting to parse Web documents must support the following features:

	* '':-webkit-autofill'' must be treated as a [=legacy selector alias=] of '':autofill''.
		<!-- expand this into a table or something if we add any more aliases -->

	* All other [=pseudo-elements=] whose names begin with the string “-webkit-”
		(matched <a>ASCII case-insensitively</a>)
		and that are not functional notations
		must be treated as valid at parse time.
		(That is, <css>::-webkit-asdf</css> is valid at parse time,
		but <css>::-webkit-jkl()</css> is not.)
		If they're not otherwise recognized and supported,
		they must be treated as matching nothing, and are
		<dfn export>unknown -webkit- pseudo-elements</dfn>.

		<a>Unknown -webkit- pseudo-elements</a>
		must be serialized in ASCII lowercase.

		<details class=note>
			<summary>What's this quirk about?</summary>

			Selectors have long had a behavior
			where a single unknown/invalid selector
			invalidates the entire selector list
			(rather than just invalidating the one complex selector it finds itself in).
			This is generally considered a legacy mistake by the WG,
			but can't be fixed at this point,
			as too many stylesheets depend on this behavior,
			intentionally or not.

			One aspect of this is that use of vendor-specific selectors
			invalidates the entire selector in other user agents
			that don't recognize them,
			and takes the entire style rule down with it.
			This has been used intentionally in the past--
			in the severely-not-recommended practice of hiding style rules from some browsers
			by making them invalid in every other browser--
			and unintentionally,
			with people styling an element
			and also applying those styles to a vendor-specific pseudo-element
			(such as the various <{input}>-related pseudos some browsers expose),
			not realizing that this hides the entire rule from other browsers.

			In addition to this more general reasoning,
			WebKit-derived user agents,
			such as Safari or Chrome,
			have an additional quirk related to their vendor-prefixed pseudo-elements,
			where any <css>::-webkit-</css>-prefixed selectors are considered valid at parse time.
			(This is probably a leftover quirk of an early CSS feature,
			since dropped,
			that intentionally treated all possible pseudo-elements as valid at parse time,
			in anticipation of a feature letting authors define their own pseudo-elements.)

			Similar to other legacy quirks,
			such as those documented in [[QUIRKS]],
			this particular vendor-specific oddity has become common enough
			that other user agents are seeing sites breaking due to them depending on it,
			accidentally or not.
			As such, since the quirk is in practical terms
			<em>required</em> to render the modern web correctly,
			specifying it and requiring it for all user agents
			ensures that today's web pages are more likely to be correctly rendered
			in user agents both current and future.

			As usual with quirks, however,
			webpages intentionally relying on this
			will be met with shaming and derision from members of the CSSWG,
			and all right-thinking web developers.
		</details>


<h2 id="visited-privacy">
Appendix C: Example Privacy-Preserving '':visited'' Restrictions</h2>

Previous attempts to protect user privacy in '':visited''
involved <a href="https://un5j2j18xhuv2emkwgjjkgb49yug.julianrbryant.com/en-US/docs/Web/CSS/Privacy_and_the_:visited_selector">complex restrictions and behaviors</a>
to "lie" about whether the link match '':visited'' or '':link'',
to reduce the chance that a hostile site
could observe what unrelated sites a user had visited
while still allowing '':visited'' to work in all cases
and help the user know what links they'd already clicked.
This is ultimately an arms race that can't be won;
there are multiple documented ways to still extract a user's browsing history
even with these mitigations.

This section describes an approach first developed and documented at
<a href="https://github.com/explainers-by-googlers/Partitioning-visited-links-history">https://github.com/explainers-by-googlers/Partitioning-visited-links-history</a>,
that partitions a user's browsing history information,
to allow '':visited'' to only match links
corresponding to navigations that the site's origin could have observed on its own.
With this, '':visited'' can be treated as a normal pseudo-class,
without any of the complex mitigations described above,
as it doesn't expose any information not already theoretically available to the site,
while still preserving as much of the <em>usefulness</em> of '':visited'' as possible for the user.

1. Let |visited history| be a [=/set=]
	containing [=tuples=] of three pieces of information:
	* a visited [=/URL=]
	* an [=/origin=] for the site that started a navigation
	* a [=/site=] for the top-level site containing the frame that started the navigation.
		(This will often be the same as the previous,
		but can differ if the user clicks a link in a iframe, for example.)

2. Whenever a navigation is triggered <em>from within a page</em>--
	e.g.,
	from the user clicking a link,
	or a script on the page initiating a navigation--
	add an entry to |visited history|
	recording the navigation's destination URL,
	the origin of the page containing the link or script,
	and the (schemeful) site of the top-level site containing that
	page (which might be the same site as the previous origin).

	Note: This allows a site to see '':visited'' information
	for links that the user has clicked
	from anywhere in that site's origin.
	In other words, any <code>A -> B</code> navigation
	where the site is A.

	Additionally, add an entry to |visited history|
	recording the destination's URL, the <em>destination's</em>
	origin, and the <em>destination's</em> site. Do this only
	for navigations from top-level frames or iframes which are
	same-origin with their top-level frame.

	Note: This allows for a site to see '':visited'' information about its own pages
	(which is already observable by the site)
	regardless of what site initiated the navigation to that page.
	In other words, any <code>A -> B</code> navigation
	where the site is B.

	Note: Notably, direct navigations triggered by the <em>user agent's</em> UI,
	such as typing into the address bar,
	clicking on bookmarks,
	or dragging a link from another program into the page,
	<em>do not</em> add a |visited history| entry.
	These can, of course,
	still add to the browser's record of visited sites
	that it uses for other purposes,
	such as suggesting URLs as the user types into the URL bar.

3. When determining if a link element should match '':link'' or '':visited'',
	only allow it to match '':visited'' if
	the link's destination,
	the origin of the page containing the link,
	and the top-level site containing the link
	match a tuple in |visited history|.

<div class=note>
	The inclusion of both page origin and top-level site
	prevents several possible privacy attacks,
	such as:

	* If history entries were <em>only</em> keyed by the starting site's URL,
		a tracking site could be embedded in a hidden iframe on multiple sites
		which triggers a navigation to a unique URL for a user on the first visit,
		and then uses many such links on subsequent visits
		to see which one had been visited,
		effectively becoming a new "third-party cookie"
		identifying the user across the web.
		By keying the history entry with the top-level site,
		this information can't be shared across different sites.

	* If history entires were <em>only</em> keyed by the top-level site's URL,
		a hostile iframe,
		perhaps included in a page as part of an advertisement,
		could observe what sites were visited from the top-level site.
		By keying the history entry with the link's own site,
		the top-level site's information can't "leak" into cross-origin iframes.
</div>


<h2 id="changes">
Changes</h2>

<h3 id="changes-2022-11">
Changes since the 11 November 2022 Working Draft</h3>

	Significant changes since the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2022/WD-selectors-4-20221111/">11 November 2022 Working Draft</a>:

	* Defined language range as BCP 47 extended language range
		(<a href="https://github.com/w3c/csswg-drafts/issues/13646"Issue  13646></a>)
	* Removed the at-risk status from '':read-write'' and '':has()''
	* Added '':popover-open'' pseudo-class.
		(<a href="https://github.com/w3c/csswg-drafts/issues/8637">Issue 8637</a>)
	* Made '':has()'' and the selector argument of '':nth-child()''/'':nth-last-child()''
		no longer forgiving.
		(<a href="https://github.com/w3c/csswg-drafts/issues/7676">Issue 7676</a>)
	* Moved the legacy single-colon pseudo-element syntax into the grammar itself.
		(<a href="https://github.com/w3c/csswg-drafts/issues/8122">Issue 8122</a>)
	* Deferred the '':local-link'' pseudo-class to Level 5.
		(<a href="https://github.com/w3c/csswg-drafts/issues/12799">Issue 12799</a>)
	* Deferred the '':interest-source'' and '':interest-target'' pseudo-classes to Level 5.
		(<a href="https://github.com/w3c/csswg-drafts/issues/12799">Issue 12799</a>)
	* Deferred the '':blank'' pseudo-class to Level 5.
		(<a href="https://github.com/w3c/csswg-drafts/issues/12799">Issue 12799</a>)
	* Deferred the <a href="selectors-5#grid-structural-selectors">grid-structural (column) selectors</a> to Level 5.
		(<a href="https://github.com/w3c/csswg-drafts/issues/12799">Issue 12799</a>)
	* Deferred the <a href="selectors-5#time-pseudos">time-dimensional pseudo-classes</a> to Level 5.
		(<a href="https://github.com/w3c/csswg-drafts/issues/12799">Issue 12799</a>)

<h3 id="changes-2022-05">
Changes since the 7 May 2022 Working Draft</h3>

	Significant changes since the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2022/WD-selectors-4-20220507/">7 May 2022 Working Draft</a>:

	* Added '':open'' pseudo-class.
		(<a href="https://github.com/w3c/csswg-drafts/issues/7319">Issue 7319</a>,
		<a href="https://github.com/w3c/csswg-drafts/issues/11039">Issue 11039</a>)
	* Disallowed [=pseudo-elements=] from '':has()'' unless explicitly allowed
		by the pseudo-element’s definition.
		(<a href="https://github.com/w3c/csswg-drafts/issues/7463">Issue 7463</a>)
	* Disallowed nesting of '':has()''.
		(<a href="https://github.com/w3c/csswg-drafts/issues/7344">Issue 7344</a>)
	* Defined matching of ''::lang("")'' and of elements not tagged with a language.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6915">Issue 6915</a>)
	* Untangled the concepts of "scoped" and "relative" selectors completely.
		(<a href="https://github.com/w3c/csswg-drafts/issues/6399">Issue 6399</a>)
		* Removed "absolutize a selector" as well,
			and just defined relative selector matching
			in terms of the anchoring element.
	* Reverted compound selector limitation on '':nth-child()''.
		(<a href="https://github.com/w3c/csswg-drafts/issues/3760">Issue 3760</a>)
	* Defined '':-webkit-autofill'' [=legacy selector alias=].
		(<a href="https://github.com/w3c/csswg-drafts/issues/7474">Issue 7474</a>)


<h3 id="changes-2018-11">
Changes since the 21 November 2018 Working Draft</h3>

	Significant changes since the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2018/WD-selectors-4-20181121/">21 November 2018 Working Draft</a>:

	<ul>
		<li>Removed the Selector profiles, marked '':has()'' as optional and at-risk instead.
			(<a href="https://github.com/w3c/csswg-drafts/issues/3925">Issue 3925</a>)
		<li>Added [[#sub-pseudo-elements]] to define [=sub-pseudo-elements=] and related terminology.
		<li>Added '':defined''.
			(<a href="https://github.com/w3c/csswg-drafts/issues/2258">Issue 2258</a>)
		<li>Added '':modal''.
			(<a href="https://github.com/w3c/csswg-drafts/issues/6965">Issue 6965</a>)
		<li>Added '':fullscreen'' and '':picture-in-picture''.
			(<a href="https://github.com/w3c/csswg-drafts/issues/3796">Issue 3796</a>)
		<li>Added '':seeking'', '':buffering'', and '':stalled'' media playback state pseudo-classes.
			(<a href="https://github.com/w3c/csswg-drafts/issues/3821">Issue 3821</a>)
		<li>Added '':muted'' and '':volume-locked'' sound state pseudo-classes.
			(<a href="https://github.com/w3c/csswg-drafts/issues/3821">Issue 3821</a> and <a href="https://github.com/w3c/csswg-drafts/issues/3933">Issue 3933</a>)
		<li>Added '':autofill''.
			(<a href="https://github.com/w3c/csswg-drafts/issues/5775">Issue 5775</a>)
		<li>Added '':user-valid''.
			(<a href="https://un5xruhmgkj46tygt32g.julianrbryant.com/Archives/Public/www-style/2015Sep/0111.html">Discussion</a>)
		<li>Defined '':is()'', '':where()'', '':has()'', '':nth-child()'', and '':nth-last-child()''
			to not be themselves invalidated when containing an invalid selector.
			(<a href="https://github.com/w3c/csswg-drafts/issues/3264">Issue 3264</a>)
		<li>Limited selectors in '':nth-child()'' and '':nth-last-child()'' to [=compound selectors=] for now.
			(<a href="https://github.com/w3c/csswg-drafts/issues/3760">Issue 3760</a>)
		<li>Clarified case-sensitive string matching by referencing string identity as defined in [[INFRA]].
		<li>Clarified that UA-provided placeholder text still triggers '':placeholder-shown''.
		<li>Rewrote '':focus-visible'' definition for clarity.
		<li>Switched reminder note in the grammar section to normative text
			describing the requirement of whitespace between <<compound-selector>>s
			when a <<combinator>> token is missing.
	</ul>


<h3 id="changes-2018-02">
Changes since the 2 February 2018 Working Draft</h3>

	Significant changes since the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2018/WD-selectors-4-20180202/">2 February 2018 Working Draft</a>:

	<ul>
		<li>Named the zero-specificity selector to '':where()''.
		(<a href="https://github.com/w3c/csswg-drafts/issues/2143">Issue 2143</a>)
		<li>Renamed '':matches()'' to '':is()''.
		(<a href="https://github.com/w3c/csswg-drafts/issues/3258">Issue 3258</a>)
		<li>Redefined '':empty'' to ignore white-space&ndash;only nodes.
		(<a href="https://github.com/w3c/csswg-drafts/issues/1967">Issue 1967</a>)
		<li>Redefined '':blank'' to represent empty user input, rather than empty elements.
		(<a href="https://github.com/w3c/csswg-drafts/issues/1283">Issue 1283</a>)
		<li>Changed the specificity of '':is()'', '':has()'', and '':nth-child()''
		to not depend on which selector argument matched.
		(<a href="https://github.com/w3c/csswg-drafts/issues/1027">Issue 1027</a>)
		<li>Dropped the '':drop()'' pseudo-classes since HTML dropped the related feature.
		(<a href="https://github.com/w3c/csswg-drafts/issues/2257">Issue 2257</a>)
		<li>Added the case-sensitive flag <code>s</code> to the attribute selector.
		(<a href="https://github.com/w3c/csswg-drafts/issues/2101">Issue 2101</a>)
		<li>Added further guidance on '':focus-visible''.
		<li>Added [[#compat]] defining ''::-webkit-'' pseudo-element parsing quirk.
		(<a href="https://github.com/w3c/csswg-drafts/issues/3051">Issue 3051</a>)
		<li>Rewrote grammar rules about where white space is allowed for clarity.
		(See [[#grammar]].)
	</ul>


<h3 id="changes-2013">
Changes since the 2 May 2013 Working Draft</h3>

	Significant changes since the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2013/WD-selectors4-20130502/">2 May 2013 Working Draft</a> include:

	<ul>
		<li>Added the '':target-within'', '':focus-within'', '':focus-visible'', '':playing'', and  '':paused'' pseudo-classes.
		<li>Added a zero-specificity '':matches()''-type pseudo-class, with name TBD.
		<li>Replaced subject indicator (''!'') feature with '':has()''.
		<li>Replaced the '':nth-match()'' and '':nth-last-match()'' selectors
			with '':nth-child(&hellip; of <var>selector</var>)'' and '':nth-last-child(&hellip; of <var>selector</var>)''.
		<li>Changed the <css>:active-drop-target</css>, <css>:valid-drop-target</css>, <css>:invalid-drop-target</css> with '':drop()''.
		<li>Sketched out an empty-or-whitespace-only selector for discussion
			(See <a href="https://github.com/w3c/csswg-drafts/issues/1967">open issue</a>.)
		<li>Renamed '':user-error'' to '':user-invalid''.
			(See <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/mid/CADhPm3v+WfeGQfBwwx8QBuiOjn2k38V_DcKW17Cm81VgZb1nbQ@mail.gmail.com">Discussion</a>)
		<li>Renamed '':nth-column()''/'':nth-last-column()'' to '':nth-col()''/'':nth-last-col()''
			to avoid naming confusion with a potential ''::column'' pseudo-class.
		<li>Changed the non-functional form of the <css>:local-link</css> pseudo-class to account for fragment URLs.
		<li>Removed the functional form of the <code>:local-link()</code> pseudo-class and reference combinator for lack of interest.
		<li>Rewrote selectors grammar using the CSS Value Definition Syntax.
		<li>Split out <a>relative selectors</a> from <a>scoped selectors</a>,
			as these are different concepts that can be independently invoked.
		<li>Moved definition of <<An+B>> microsyntax to CSS Syntax.

		Issue: Semantic definition should probably move back here.

		<li>Added new sections:
			<ul>
				<li>[[#data-model]]

				Issue: Need to define tree for XML.
				<li>[[#api-hooks]]
					<ul>
						<li>Note that earlier versions of this section defined a section on <dfn noexport id="evaluating-selectors">evaluating a selector</dfn>,
						but that section is no longer present.
						Specifications referencing that section should instead reference
						the algorithm to <a>match a selector against a tree</a>.
					</ul>
				</li>
			</ul>
		<li>Removed restriction on combinators within '':matches()'' and '':not()'';
			see <a href="https://un5xruhmgkj46tygt32g.julianrbryant.com/Archives/Public/www-style/2014Jan/0607.html">discussion</a>.
		<li>Defined <a>specificity</a> of a <a>selector list</a>. (Why?)
		<li>Required quotes around '':lang()'' values involving an asterisk;
			only language codes which happen to be CSS identifiers can be used unquoted.
	</ul>

	Note: The 1 February 2018 draft included an inadvertent commit of unfinished work;
	2 February 2018 has reverted this commit (and fixed some links because why not).


<h3 id="changes-2012">
Changes since the 23 August 2012 Working Draft</h3>

	Significant changes since the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2012/WD-selectors4-20120823/">23 August 2012 Working Draft</a> include:

	<ul>
		<li>Added '':placeholder-shown'' pseudo-classes.
		<li>Released some restrictions on '':matches()'' and '':not()''.
		<li>Defined fast and complete Selectors profiles (now called “live” and “snapshot”).
		<li>Improved definition of <a>specificity</a> to better handle <a>:matches()</a>.
		<li>Updated grammar.
		<li>Cleaned up definition of <<An+B>> notation.
		<li>Added definition of <i>scope-relative</i> selectors, changed <i>scope-constrained</i> to scope-filtered for less confusion with scope-contained.
		<li>The '':local-link()'' pseudo-class now ignores trailing slashes.
	</ul>


<h3 id="changes-2011">
Changes since the 29 September 2011 Working Draft</h3>

	Significant changes since the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2011/WD-selectors4-20110929/">29 September 2011 Working Draft</a> include:

	<ul>
		<li>Added language variant handling per RFC 4647.
		<li>Added scoped selectors.
		<li>Added <css>:user-error</css> (now called '':user-invalid'').
		<li>Added <css>:valid-drop-target</css>.
		<li>Changed <a>column combinator</a> from double slash to double pipe.
	</ul>


<h3 id="changes-level-3">
Changes Since Level 3</h3>

	Additions since <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/selectors-3/">Level 3</a>:

	<ul>
		<li>Extended '':not()'' to accept a selector list.
		<li>Added '':is()'' and '':where()'' and '':has()''.
		<li>Added '':scope''.
		<li>Added '':any-link''.
		<li>Added '':target-within'', '':focus-within'', and '':focus-visible''.
		<li>Added '':dir()''.
		<li>Expanded '':lang()'' to accept wildcard matching and lists of language codes.
		<li>Expanded <css>:nth-child()</css> to accept a selector list.
		<li>Merged in input selectors from <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/css-ui-3/">CSS Basic User Interface Module Level 3</a> and added back '':indeterminate''.
		<li>Added '':user-invalid''.
		<li>Added case-insensitive / case-sensitive attribute-value matching flags.
	</ul>


<h2 id="acknowledgements">
Acknowledgements</h2>

	The CSS working group would like to thank everyone who contributed
	to the <a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/css3-selectors">previous Selectors</a> specifications over the years,
	as those specifications formed the basis for this one.
	In particular, the working group would like to extend special thanks
	to the following for their specific contributions to Selectors Level 4:
	L. David Baron,
	Andrew Fedoniouk,
	Daniel Glazman,
	Ian Hickson,
	Grey Hodge,
	Lachlan Hunt,
	Anne van Kesteren,
	Jason Cranford Teague,
	Lea Verou


<h2 class=no-num id=privacy>Privacy Considerations</h2>

	<ul>
		<li>
			The '':visited'' pseudo-class can expose information about which sites a user has previously visited,
			if the UA is not careful to screen from scripting any information that would reveal
			which elements match it.
		<li>
			The '':autofill'' pseudo-class can expose whether a user has interacted with this form before;
			however the same information can be derived by observing how quickly the form is filled out.
	</ul>

	<h2 class=no-num id=security>Security Considerations</h2>
	
	The <a href="#privacy">Privacy Considerations</a>
	could also be considered to affect Security.

<wpt hidden title="Features from higher-level specs">
	heading.html
	invalidation/heading-pseudo-class-in-has.html
	parsing/parse-heading.html
	parsing/parse-state.html
</wpt>

<wpt hidden title="Features from other specs and unrelated tests">
	backdrop-input-crash.html
	first-letter-flag-001.html
	first-line-bidi-001.html
	first-line-bidi-002.html
	floating-first-letter-05d0.html
	floating-first-letter-feff.html
	invalidation/part-pseudo.html
	link-sharing-crash.html
	parsing/parse-part.html
	parsing/parse-slotted.html
	quirks-mode-import.html
	selection-image-001-no-selection-noref.html
	selection-image-001-noref.html
	selection-image-001.html
	selection-image-002.html
	spurious-brace-crash.html
</wpt>