I mean, CSS validation is useful only to detect parsing errors during the development process, but nothing more. To be honest, however, this is something that browsers are able to do by themselves without checking the style sheet with an external resource, because if a CSS file has some syntactic errors, well, it doesn't work as expected. Period. Valid is a different concept. CSS is basically a language with a strong backward compatibility design, so the syntactic differences between versions 2.1 and 3 are only theoretical, because a CSS 2.1 file is also a valid CSS 3 file and vice-versa. So my point is: CSS validation should not be used to show out that your site is cool or standard or just to show this badge that claims "Valid". This is a fad, and it's quickly disappearing. Just perform validation only if you're in doubt about the syntax used. Don't be afraid of using vendor extensions or something like that: they don't hurt! What's more, that can't hurt your visitors.