CSS version selectors?

Does anyone have any thoughts on CSS version selectors? I don't know 
about this examples' syntax, but I envisioned something that would only 
work if an agent supported a full specification:

html::css2 #main {
   color: #fff;
   background-color: #fff;
   text-shadow: #000 0px 0px 5px;
}

The point is that it would avoid the white-on-white text that would come 
from most browsers that ignore text-shadow. I realize Safari can 
currently support this, but the idea is for something like the language 
attribute of a script element. Subsequent versions css2-1, css3, css4, 
etc. could also be supported.

Of course, this has the option of being abused because a browser-maker 
could implement the version selector without fully implementing the rest 
of the specification. I'm looking for a more standard approach to 
conditional CSS than comment hacks or selector differentiation. Both of 
those approaches have higher possibility of failure in new browsers.

Thanks,
James Craig

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