some browser-specific CSS3 implementations had caused some invalid cod

Yes, but who cares. There is a difference between someone who can't code valid, and someone who is using a bit of CSS so those who use real browsers can get a slightly better experience.

It is totally different to say valid HTML - that, you have no excuse to get wrong (except when clients edit it haha). If the CSS invalidation is ONLY because of CSS3, or.... hmm, say IEpngfix with behavior, or expressions to fix IE min/max width - then that's fine. Is it perfect? No. But do we live in a perfect world? Not til IE goes die in a hole