Sass

Sass stands for syntactically awesome style sheets and is a mature, stable, and powerful CSS extension language. With Sass, web designers can work smarter and faster with CSS by providing a more elegant syntax for CSS and implementing various features that are useful for creating manageable stylesheets at scale. Some of these features include adding nested rules, variables, mixins, selector inheritance, and more. Sass has been active development since 2006, and continues to provide value to the web designer community until this day. It's released free and open-source under the MIT license.