"A @property is invalid if it occurs in a stylesheet inside of a shadow tree, and must be ignored."A https://drafts.css-houdini.org/css-properties-values-api/#at-property-rule
<rdar://problem/104221943>
Pull request: https://github.com/WebKit/WebKit/pull/8617
Committed 258880@main (997d987fd5b6): <https://commits.webkit.org/258880@main> Reviewed commits have been landed. Closing PR #8617 and removing active labels.