[csswg-drafts] [css-pseudo-4] Mark unimplemented CSSPseudoElement features at-risk (#3540)

fantasai has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-pseudo-4] Mark unimplemented CSSPseudoElement features at-risk ==
Gecko is implementing parts of the CSSPseudoElement interface to integrate with Web Animations, so those parts should stay. If there aren't strong use cases for other parts, we should either drop them (if we're not sure of the design) or mark them at-risk (if we're sure they're correctly designed, but not sure if they're needed).

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/3540 using your GitHub account