[odedta]

I disagree with what kovid said about validation being a waste of time and I agree with skreutzer. None-conformance to standards is what got us in this mess in the first place, each reader requires it's own quirks.
I am all for validating any document you make, sticking to standards when writing code is more than good practice, it's crucial.

On the subject matter: I create an ePub 3 with <u> tags and it passed validation with no errors. View attached.