https://drafts.css-houdini.org/css-properties-values-api/#at-property-rule
<rdar://problem/103493363>
Pull request: https://github.com/WebKit/WebKit/pull/7826
Committed 258067@main (8c6f3e96197b): <https://commits.webkit.org/258067@main> Reviewed commits have been landed. Closing PR #7826 and removing active labels.