The problem lies in the vendor prefix, e.g. -moz-, being valid, but the validator having no knowledge of whether the vendor property, as written, is valid. These vendor specific properties allow testing without the danger of future breakage. They are not part of the specification beyond the prefix. Ergo, they cannot be validated against the specs.
And, yeah, take the validation links off. Visitors don't care, and presumably you've discussed the issue with your client, should he happen to care.