Resync html/semantics/embedded-content/the-iframe-element from upstream WPT.
Pull request: https://github.com/WebKit/WebKit/pull/15840
Committed 266081@main (610fb6e0431e): <https://commits.webkit.org/266081@main> Reviewed commits have been landed. Closing PR #15840 and removing active labels.
<rdar://problem/112311088>