Before this change, we assigned the style sheet's location *after* its
content rules were parsed and added to it. This meant any `@import`s
would try to fetch their style sheet before they knew the URL they
should fetch it relative to.
We were previously crashing when invoking 'scroll to the fragment' on
such documents as it was unable to find the active document. This is
as a result of our AD-HOC implementation not setting up the document
fully to mark it is active before running the parser.
Fixes a crash on https://tweakers.net.