The changes should be trivial.

The third_party changes are up for review in
https://github.com/servo/rust-cssparser/pull/277 (and of course I'll
land with a bump to 0.28 rather than the override after that gets r+'d).

The basic idea is that with this we have the actual start offset of the
rule, so we wouldn't include html comments or other invalid stuff we
discard during sanitization in bug 1680084. But that's a separate
change.

Differential Revision: https://phabricator.services.mozilla.com/D98677