This is a little awkward: The spec requires when loading media documents
or ones that don't have a DOM, that we "act as if the user agent had
stopped parsing document" which means following this algorithm. Only a
few steps require an HTMLParser, but those that do, involve reaching
into its internals. The simplest solution I could think of (other than
duplicating this fairly hefty function) is making it static and taking
a Document and optional HTMLParser as parameters.
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