Originally Posted by kiwidude
CSS validation I don't recall ever coming up on this thread (beyond the wish for detecting unused styles). I vaguely recall some discussion in the Sigil forum but I never paid any notice to it. It would have to be an offline option so if you have a particular candidate tool I'm happy to give it a quick whirl. Though of course it may suffer the same pitfalls as those other tools above and again be something that would be best implemented in Sigil...
Well, the w3 validation can be downloaded for offline use. The jar file can be downloaded and called with options. At least, that is what W3
I tend to agree with the arguments of EpubCheck and FlightCrew, although I tend to solve all the errors.