Less is an extended variant of CSS. Its accompanying website, lesscss.org, really says it all very concise and I only want to add a +1 to the whole idea of mixins for CSS and hope something like this will inspire those writing the CSS specifications.

You can either include a JS file that parses your rules written in less on page load, or you could compile it on beforehand with for example lessphp.

Bonus: there is syntax highlighting in vim available.