CSS Error & Warning free: ! or ?
As I've started "W3C compliant CSS"-issue (http://drupal.org/node/70296) as a wish, it was the first one in the chain of validitaion.
The main critism of warning-free css-validation is the "inherit" argument, which changes nothing visible, but is needed by W3C-validation (warning free).
In my opinion there is a difference between an "explicit defined default" and an "undefined default", cause the "explicit defined" says: Yes, I'm sure, that I want to do this.
As we are experienced webusers, we know that the warning of "undefined color" doesn't make visible sense. But it's one of my most-hated questions customers asks me twice a month: "Why does your site so much validation warnings, although you are working active for accessibility?".