A pre-processor is a tool for interpreting CSS before sending out to the client. This means that the developer can add CSS in a more complex syntax, and then send it to the pre-processor to convert it into standard CSS code.

CSS Crush is a PHP based CSS pre-processor that automagically handles vendor prefixes, CSS variables, data-uris, minification and much more.

It aims to alleviate many of the hacks and workarounds necessary in modern CSS development.