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.
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