CSS2 Validation error with Gradient

The following is my CSS code:

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table th{  font-family:arial; font-size:9pt;  color:#ffffff; background: -moz-linear-gradient(#b9cdde, #7c98ae);  border: 1px solid #ffffff; }

table th:last-child{  background: -moz-linear-gradient(#729cc3, #35699a); }

I get a validation error with CSS which says

Value Error : background Too many values or values are not recognized : -moz-linear-gradient(#b9cdde,#7c98ae) -moz-linear-gradient(#b9cdde,#7c98ae)

Does someone have any idea as to why this might be happening?

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Vendor-specific selectors such as -moz-linear-gradient are not part of the official CSS2 specification, so when the validator finds then, it will throw an error. Personally, I don't mind if stuff like this doesn't validate - it's only a nice, cleanly written gradient.