PostCSS
A tool for transforming CSS with JavaScript
...These plugins can do a great number of things: transpile future CSS syntax, lint your CSS, support variables and mixins, and so much more. PostCSS works by taking a CSS file and providing an API to analyze and modify its rules (through its transformation into an Abstract Syntax Tree). The API can then be used by plugins to do a lot of useful things.
With PostCSS, you can increase code readability by adding vendor prefixes to CSS rules; convert modern CSS so it's understood by more browsers; and avoid errors in your CSS via stylelint, a modern CSS linter. ...