It's not actually implemented: https://searchfox.org/wubkat/rev/dd3c292ca93c8499aab292ab26e6b71fcf5665b1/Source/WebCore/css/SelectorChecker.cpp#1124-1126 ...and not in the spec either: https://github.com/w3c/csswg-drafts/issues/3596
Pull request: https://github.com/WebKit/WebKit/pull/2695
Committed 252786@main (4f6fdd5fa416): <https://commits.webkit.org/252786@main> Reviewed commits have been landed. Closing PR #2695 and removing active labels.
<rdar://problem/97556629>