[css-lists][css-pseudo] Add ::marker to interface CSSPseudoElement

[MatsPalmgren]

https://drafts.csswg.org/css-pseudo-4/#csspseudoelement

Firefox is shipping support for ::marker in v68, including animation support.
Safari supports it since v11 according to caniuse.

[fantasai]

Added. Agenda+ to confirm.

[css-meeting-bot]

The CSS Working Group just discussed Add ::marker to interface CSSPseudoElement, and agreed to the following:

• RESOLVED: Add ::marker to interface CSSPseudoElement

The full IRC log of that discussion

<dael> Topic: Add ::marker to interface CSSPseudoElement
<dael> github: https://github.com//issues/3763
<dael> fantasai: This is straightforward. HAve a pseudo element similar to before/after. Just add it to the pseudo element interface
<dael> plinss: Objections:
<dael> RESOLVED: Add ::marker to interface CSSPseudoElement