It has been gone from HTML for a while now: https://github.com/whatwg/html/commit/7512c0d2375e573dfc5c3341662c887237a44459.
<rdar://problem/107416609>
Pull request: https://github.com/WebKit/WebKit/pull/12173
Committed 262431@main (5b2c72f4aa1f): <https://commits.webkit.org/262431@main> Reviewed commits have been landed. Closing PR #12173 and removing active labels.