CSS Validation Question

Hi,

I was validating my CSS file and this line was reporting as an error...

> Line: 407 Context : #button_nav li
> Invalid number : display inline-block is not a display value : inline-block
> ...http://www.w3.org/TR/REC-CSS2/visuren.html#propdef-display


Here is the code...

> #button_nav li {
>   display: inline-block;
>   list-style-type: none;
>   color: #FFF;
>   white-space: nowrap;
>   }

Is it an error. I looked at all the acceptable values and "display:
inline-block;" seems to be valid.
http://www.w3.org/TR/CSS21/visuren.html#propdef-display

Is this because the validator is built on CSS 2 and not CSS 2.1?

Thanks, Doug