Nope. Your only weapon is called testing. I've had pages that validate render differently on different browsers.
Running a validation program is only one sanity check. If something doesn't validate, I check it out, but I don't fix it if it ain't really broke. It's like getting warnings back from a compiler, it is simply telling you that something might be wrong.
Yeah, I would. I would also explain why they shouldn't care. Being willing to tell them such things, and being able to explain why, is the reason that I was usually the highest paid contract programmer at those companies.