The hubbub bindings set document.characterSet but we don't use it for anything.
html5ever doesn't parse charsets from <meta> tags yet, and I'd rather add that
as part of adding actual support for other encodings (which Servo never had
with hubbub).
Switching to html5ever also changes attribute parsing slightly (bringing it in
line with the current spec) and this breaks some of the <img srcset> tests. We
don't support this feature at all.
Some other things change due to bugs in hubbub not present in html5ever.
