This necessitated making HTMLParser ref-counted, and having it register
itself with Document when created. That makes it possible for scripts to
add new input at the current parser insertion point.
There is now a reference cycle between Document and HTMLParser. This
cycle is explicitly broken by calling Document::detach_parser() at the
end of HTMLParser::run().
This is a huge progression on ACID3, from 31% to 49%! :^)
Previously it would accept any DOMString, as we didn't support enums at
the time. Now it will only accept what's specified in the
DOMParserSupportedType enum.
This also adds spec comments to DOMParser::parse_from_string.
This allows you to invoke the HTML document parser and retrieve a
document as though it was loaded as a web page, minus any scripting
ability.
This does not currently support XML parsing.
This is used by YouTube (or more accurately, Web Components Polyfills)
to polyfill templates.
