means nothing. Valid CSS does not guarantee a good user experience, the layout is correct, or the site even functions at all. This nonsense must end. Jeffrey Way's article But It Doesn't Validate is another rant on the topic. I would definitely recommend you read it.
never, ever, ever compromise the use of the latest doctype, CSS3 techniques and selectors for the sake of validation Jeffrey Way, But It Doesn't ValidateThe argument of using the validation to check for syntax errors (typos, missing semicolons, etc) is moot because I work with SASS and it will not compile into CSS if there is a syntax error. Also, it gives you a nice little message telling you the problem.
This is ultimately up to you. If you want to be stuck in the past ignoring new technology completely, have at it but don't force it on others.