[CSS] Extending media queries

Hello,

I just read this (draft of) proposition from Webkit about extending the 'Media Queries' so that it can reflect the implementation of some (currently proprietary) properties introduced by Webkit (http://webkit.org/specs/MediaQueriesExtensions.html)

I wonder why we don't have a simple ...
    @media (implements: position 2.1) {
        ...
    }
... to know if the browser implements the 'position' property as the CSS2.1 spec ask it. It would be more generalisable than juste adding some new values (transform, transform-2d, ...).

Here's a bit more about how I imagine my proposition :

@media (implements: position) { ... } 
>>> True if the browser has a CSS property called 'position', even if this property is not standardized or W3C non-conformant.

@media (implements: position 2.1) { ... } 
>>> True if the browser mostly supports the 'position' property as the W3C defines it. If the browser is build before the standard is completely OK, then it should return False, even if he thinks the standard will not change and that he's ok.

@media (implements: position 2.1 strict) { ... } 
>>> True if the browser fully (whitout any known bug) supports the 'position' property as the W3C defines it. If the browser is build before the standard is completely OK, then it should return False, even if he thinks the standard will not change and that all is ok.

@media (implements: position 3 drafts) { ... } 
>>> True if the browser mostly supports the 'position' property as the W3C defines it in a draft posted in the CSS 2.1 section. If the browser is build before the standard is completely OK, then it may return True.

Note: if 'strict' and 'drafts' are both defined, the 'drafts' should be ignored.

Here's the syntax I thus propose :
(implements: <CSSProperyName> <CSSVersion>? <params>*)

What do you think of it?
Fremy