I guess, it's more about the JS stuff than CSS for him. I listened to a talk of him once where he denounced unfree JS code whose execution the user basically can only prevent using whitelisting. And even so, some pages may become completely unusable if JS is disabled.
What he (or rather, someone from the FSF) actually did, was creating a Firefox plugin which would detect unfree JS, disable its execution and automatically send a complaint to the host site (as long as that's possible without JS). At least that's what he told.
CSS is (imho) not code just as HTML is not really code (although I know HTML5/CSS3 being Turing complete, but well). JS is in my eyes the much bigger problem (and the one I know RMS is actually concerned about).