Sass (which stands for Syntactically Awesome Style Sheets) is the most mature, stable, and powerful professional grade CSS extension language in the world.
Saas includes various features such as variables, nested rules, mixins, inline imports, built-in functions to manipulate color and other values, all with a fully CSS-compatible syntax. Sass is a W3C standard established in 1996.
Links of Interest
On the Sass Bascis Web page you will find an overview of the following topics: