These new methods can parse HTML with declarative shadowdom. This is being specced here: https://github.com/whatwg/html/pull/9538
<rdar://problem/115345128>
FYI: HTML standard change has landed.
Pull request: https://github.com/WebKit/WebKit/pull/20874
Committed 271423@main (25a4ff15329b): <https://commits.webkit.org/271423@main> Reviewed commits have been landed. Closing PR #20874 and removing active labels.