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Excellent little demo.

Shows how long I've been out of touch with CSS. Didn't know stuff like variables or rounded edges was supported.

The variables appear to be a SASS construct, not CSS.

SASS compiles to CSS.

And it inlines the variables as constants at 'compile time'.

Rounded corners are supported in Webkit Gecko and Presto, and not in Trident of course.

Variables are provided by SASS, a CSS pre-processor language/framework.