"The syntax of a <style-feature> is ... the same as for a valid declaration, ..." which apparently includes !important.
<rdar://problem/123374708>
Pull request: https://github.com/WebKit/WebKit/pull/24876
Committed 275104@main (7839f9caaeec): <https://commits.webkit.org/275104@main> Reviewed commits have been landed. Closing PR #24876 and removing active labels.