62087dd417
I didn't put this behind a pref because it was a bit annoying to do with the parser changes while keeping it fast, and because, at least nesting wise, this is unlikely to cause compat issues. There are some complexities for @scope, which I think I got right. Fixed the tests not to depend on @scope necessarily per the discussion in https://github.com/web-platform-tests/interop/issues/697. Differential Revision: https://phabricator.services.mozilla.com/D222817