Adds support for the following HTML elements to the HTML Processor:
- AREA, BR, EMBED, KEYGEN, WBR
- Only the opening BR tag is supported, as the invalid closer `</br>`
involves more complicated rules, to be implemented later.
Previously, these elements were not supported and the HTML Processor
would bail when encountering them. With this patch it will proceed to
parse an HTML document when encountering those tags as long as other
normal conditions don't cause it to bail (such as complicated format
reconstruction rules).
Props jonsurrell, dmsnell
Fixes#60283
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