The context of the questions is important - they can tell you what someone knows and what they don't. I don't think the idea is "don't hire anyone who is unaware of the consequences of serving pages as application/xhtml+xml".
Plus if you are secure in your abilities you should be able to say "no, I'm not quite sure why that could be bad" without feeling like the world is ending. We all have stuff to learn.
Plus if you are secure in your abilities you should be able to say "no, I'm not quite sure why that could be bad" without feeling like the world is ending. We all have stuff to learn.