The subselectors that are not legal in the context will just reduce to nothing.
Created attachment 442802 [details] Patch
Created attachment 442803 [details] Patch
Created attachment 442827 [details] Patch
Committed r285054 (243697@main): <https://commits.webkit.org/243697@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 442827 [details].
<rdar://problem/84824275>