[selectors] The grammar for :host() and other compound-selector-taking things should probably forbid pseudo-elements

[emilio]

I just realized that WebKit (but not Blink nor Gecko) parse the following selector as valid:

:host(::before)

(or :host(::before)::before for that matter).

I was going to file a WebKit bug, but then realized that the grammar technically allows it. This also applies to the grammar of :is / :where (old :matches) etc.

I think that's wrong and pseudo-elements should not be allowed in nested selectors. WebKit itself doesn't parse, e.g., :not(::before), so they're not consistent about it.