The coolest CSS framework I know of is LESS . It allows you to add variables, inheritance, arithmetic operations and other coolness using syntax like CSS - and it is βcompiledβ to regular CSS using the build step or server plug-in.
Like C in C ++, the LESS syntax is backward compatible with CSS, so you can simply rename all your .CSS files to .LESS and start adding LESS functionality without learning a whole new language.
As comments to the comments below, Rails and PHP support LESS through server plugins. The beta version also has an ASP.NET port . There is also support for Django . And ColdFusion . And so on...
Note that using LESS (as a Ruby executable that spits out CSS files) can be as simple as running it to compile .LESS files into .CSS as part of your build process. The build machine should be able to run Ruby, of course, unless you use one of the plugins that ported the LESS compiler to this natural platform code.
Note that xCSS offers similar features for LESS - I have not heard of xCSS. AFAIK LESS is more popular, but others in the community here can weigh it, as popularity is changing rapidly!
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