CSS3 Arithmetic
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I have been using and learning CSS3 a fair bit of late and enjoying its many capabilities.  Right now I am wondering if it is possible to setup a CSS rule that assigns a block element width conditionally.  The sort of thing I am after - if the screenwidth is less than, say 500px, use a width of 320px otherwise use a width of, say, 80%, of screen size.

Yes, I realize I could do this sort of thing through JavaScript - just wondering if there isn't a more elegant CSS3 approach.