Sass (Syntactically Awesome StyleSheets)

CSS with superpowers 

Sass is the most mature, stable, and powerful professional grade CSS extension language in the world.

Ref:http://sass-lang.com

Ref:SASS用法指南

Ref:Sass vs. LESS vs. Stylus: Preprocessor Shootout

Ref:LESS介绍及其与Sass的差异