Bug in background-clip property?

Hello,

I don't know if this has been reported before or not, but the following css
command: *background-clip: content-box;* is not validating in the CSS 3
validator:
*Error: "content-box is not a background-clip value : content-box
content-box*"
However, according to the W3C documentation on
CSS3<http://www.w3.org/TR/css3-background/#the-background-clip>,
"content-box" is a valid value for that property.
Any thoughts on this?
Thanks in advanced,

Francisco de Azevedo