CSS preprocessor is a program that lets you generate CSS from the preprocessor's own unique syntax.
Sass makes CSS fun!
Home: https://sass-lang.com
Docs: https://sass-lang.com/documentation
GitHub: https://github.com/sass/sass
A dynamic stylesheet language.
Home: http://lesscss.org
Docs: http://lesscss.org/#overview
GitHub: https://github.com/less/less.js
Expressive, robust, feature-rich CSS language built for nodejs.
Home: http://stylus-lang.com
Docs: http://stylus-lang.com/docs
GitHub: https://github.com/stylus/stylus
A tool for transforming CSS with JavaScript.
Home: https://postcss.org
Docs: https://git.io/Je3xT
GitHub: https://github.com/postcss/postcss