ecss is a fully compatible CSS variant. mod_ecss (the ecss apache module) has been in production use since May 2007 serving millions of requests per day. Besides syntax-checking and compressing existing CSS, ecss provides the following extensions:

• NestedRulesets
• Variables
• Themes
• IncludeFiles
• Evaluation
• UserAgentProperties

Here's a brief example illustrating some of ecss's features:

@include "other-stuff/ads.ecss";
$width: 800;
$width_px: eval($width _ px);
$horiz_padding: eval(floor($width * 0.018));
$horiz_padding_px: eval($horiz_padding _ px);
$vertical_padding: eval(floor($horiz_padding / 6));
$vertical_padding_px: eval($vertical_padding _ px);
#myproject {
  #menu {
    ul, ol {
      list-style: none;
      li {
        height: eval($height _ em);
        padding: 0;
      }
      a {
        color: #aabbcc;
        color(msie 7.0): #ddeeff;
      }
    }
  }
}

More stuff to read:

• InstallationGuide
• Gotchas
• History