-moz-border-radius validation

At the moment, I have given up on curved borders so
that my CSS will validate.  I am also eagerly awaiting
CSS 3 being implememnted by UAs so I can use curved
borders.  However, I can not help but wonder why the
CSS validator refuses to validate these proprietary
commands when they are a part of the CSS 2.1
specification.  In particular, -moz-border-radius is
listed in the CSS 2.1 Specification as a known vendor
specific property that follows the specifications for
vendor specific properties.  I do not understand why
something defined in the specification can be deemed
invalid.  Any information you could share to help
explain this situation would be most appreciated.

Thank you,
Zvi Effron