[css-color-4] Split color conversion into two stages, #10211

Author: svgeesus

css-color-4/Overview.bs

@@ -4699,7 +4699,7 @@ Simple Alpha Compositing</h3>
 <h2 id="color-conversion" algorithm="to convert a color">
 Converting Colors</h2>
 
-<wpt title="This section provides an algorithm used later, it does not need tests."></wpt>
+<wpt title="This section provides algorithms used later, it does not need tests."></wpt>
 
 Colors may be converted
 from one color space to another and,
@@ -4709,17 +4709,28 @@ and that each color space can represent out of gamut colors,
 then (subject to numerical precision and round-off error)
 the two colors will look the same and represent the same color sensation.
 
-To convert a color |col1| in a source color space |src|
-with white point |src-white|
-to a color |col2| in destination color space |dest|
-with white point |dest-white|:
+<div algorithm>
+To <dfn export>prepare a color |col1| for conversion</dfn>:
 
 <ol>
+	<li id="powerless-to-missing">Change any [=powerless component=]s in |src| to [=missing component=]s</li>
 	<li id="convert-polrect">If |src| is in a <a>cylindrical polar color</a> representation,
-	first convert |col1|
+	convert |col1|
 	to the corresponding <a>rectangular orthogonal color</a> representation
 	and let this be the new |col1|. 
-	
+</ol>
+</div>
+
+<div algorithm>
+To <dfn export>convert a color</dfn> |col1| in a source color space |src|
+with white point |src-white|
+to a color |col2| in destination color space |dest|
+with white point |dest-white|,
+where |src| and |dest| are <em>different</em>:
+
+<ol>
+	<li>prepare |col1| for conversion</li>
+
 	<li id="convert-missing">Replace any [=missing component=] with zero.</li>
 
 	<li id="convert-tolinear">If |src| is not a linear-light representation,
@@ -4756,6 +4767,7 @@ with white point |dest-white|:
 	convert from |dest-rect| to |dest|, and let this be |col2|.
 	This may produce [=missing component=]s.
 </ol>
+</div>
 
 
 <!--
@@ -7838,7 +7850,10 @@ Changes</h2>
 	<a href="https://un5gmtkzgkj46tygt32g.julianrbryant.com/TR/2025/CRD-css-color-4-20250424/">Candidate Recommendation Draft of 24 April 2025</a></h3>
 
 <ul>
-	<!-- to 26 March 2026 -->
+	<!-- to 31 March 2026 -->
+	 <li>Split color conversion into two stages
+		(<a href="https://github.com/w3c/csswg-drafts/issues/10211">Issue 10211</a>)
+	 </li>
 	<li>Clarified how system colors react to the used color scheme
 		(<a href="https://github.com/w3c/csswg-drafts/issues/13719">Issue 13719</a>)</li>
 	<li>Updated abstract to mention color interpolation and gamut mapping.</li>