I'm in the process of validating all my HTML, CSS, etc. Using the W3C validators, I have acheived valid HTML. And, when I plug the CSS right into their form, the CSS comes back valid.
However, when I just try to enter my site's URL into the CSS validator, I get the following message:
Please, validate your XML document first!
Element type "null" must be followed by either attribute specifications, ">" or "/>".