Switch to skip CSS gauntlet; allow rect(40px, 200px, 150px, 30px)

More annoying quirks with that gauntlet regex, this time blocking rect().

[@teoli2003 suggested removing the test on styles altogether](https://bugzilla.mozilla.org/show_bug.cgi?id=776603#c23). I'm not sure that's easily done, but I've also added a `skip_css_gauntlet` flag that could let us get around what look like repeat issues in MDN. It defaults to `False`, so should be backwards compatible if other users rely on it.