For example, this bit of CSS fails and I could not find a way for the validator to accept it:
background: -webkit-gradient(linear, left top, left bottom, from(#292929), to(#9f9f9f));
background: -moz-linear-gradient(top, #292929, #9f9f9f);
Unfortunately for some elements like background vendor prefixes in values are still being needed.
They are now generating warning, as values can't be validated.