Tree-Structural pseudo-classes and pseudo-elements

[annevk]

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

Do they never match or do they never parse?

I think it's the latter, though per https://drafts.csswg.org/selectors-4/#pseudo-element-states I guess a different specification could change that? Having multiple specifications rule over whether a selector parses does not seem great. Can that be changed somehow so there is more centralized authority on that question?

[nt1m]

@emilio suggested in #9795 that the latter is better so @supports selector() can reflect it. I tend to agree.