HTML API: Reconstruct active formatting elements

[dmsnell]

Trac ticket: Core-65383

Reconstruction of #6982 and #6983 from trunk.

[github-actions]

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[dmsnell]

@sirreal I had Codex attempt to merge the two prior PRs into this, as well as rebuild it from trunk. Not yet have I verified if it did it properly.

I did notice in reviewing the older versions how valuable it will be to have in_body_any_other_end_tag() as a separate method.

[github-actions]

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