This should allow us to add a Element::attribute which returns an
Optional<String>. Eventually all callers should be ported to switch from
the DeprecatedString version, but in the meantime, this should allow us
to port some more IDL interfaces away from DeprecatedString.
Some websites (like Reddit) like to instantiate "components" by setting
innerHTML to a huge chunk of stuff. Sometimes those huge chunks of stuff
contain inline style sheets (i.e `<style>` elements).
Before this change, we would end up parsing the CSS in those elements
multiple times, because we had no way of knowing that we were within
a fragment parser's temporary document.
This patch avoids the extra CSS parsing work by adding adding a flag to
Document that tells us it's being used by the fragment parser. Then, we
simply avoid parsing CSS for style elements in such documents. The CSS
then gets parsed immediately upon insertion into the proper DOM.
