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.