In late 2009 I wrote an article explaining that Vendor-specific extensions are invalid CSS. They are also a fact of life if you want to be able to use new CSS features, which most of us do.
As I explain in the article, the CSS Validator reporting vendor-specific extensions as errors is correct, but it does make it difficult to find any real errors (typos, syntax errors, etc.) among all the “Property -webkit-border-radius doesn't exist� and similar messages. And as we add more and more vendor-specific selectors in order to use CSS3, this situation gets worse.
I suggested that the CSS Validator could somehow let you know if the errors are caused by vendor-specific extensions or if they are actual syntax errors. Well, something similar to that will soon be possible since CSS Validation Will Soon Be More Realistic.
Posted in CSS.