Validation is supposed to happen at computed value time only. https://drafts.css-houdini.org/css-properties-values-api/#parsing-custom-properties
<rdar://problem/103629699>
Pull request: https://github.com/WebKit/WebKit/pull/7999
Committed 258240@main (e78366ee2460): <https://commits.webkit.org/258240@main> Reviewed commits have been landed. Closing PR #7999 and removing active labels.