The HTML API HTML processor does not yet support all tags. Many tags (e.g. list elements) have some complicated rules in the [https://html.spec.whatwg.org/#parsing-main-inbody "in body" insertion mode].
Implementing these special rules is blocking the implementation for a catch-all rule for "any other tag" because we need to prevent special rules from being handled by the catch-all.
Any other start tag
Reconstruct the active formatting elements, if any.
Insert an HTML element for the token.
…
This change ensures the HTML Processor fails when handling special tags. This is the same as existing behavior, but will allow us to implement the catch-all "any other tag" handling without unintentionally handling special elements.
Additionally, we add tests that assert the special elements are unhandled. As these tags are implemented, this should help to ensure they're removed from the unsupported tag list.
Props jonsurrell, dmsnell.
Fixes#60092.
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