We already don't validate. There's no point to trying to conform to a validator when the spec/validator is wrong. And we already have cases like that.
I think we should try to validate though mostly for future proofing...
Anyways, technically you could already use scoped anyways. Just add the scoped attribute. Don't use css that applies outside the content area. And then it'll validate, it'll work in browsers, and when browsers actually implement scoped they'll start restricting the scope.
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But after you've dealt with all the XSS issues; you've opened up the ability to completely destroy the UI from within WikiText. In ways even worse than the tricks attempting to simply cover the whole UI with a div. Those tricks being ones you could technically eliminate by using overflow+relative on the content area and disallowing position: fixed; (The only thing in the way of that right now is WP's stupid page icon hack).
I think if we restricted css to templates that only trusted admins can edit then these problems goes away somewhat no?