PostCSS is a tool for transforming CSS with JS plugins. The growing ecosystem of PostCSS plugins can add vendor prefixes, support variables and mixins, transpile future CSS syntax, inline images, and more.
PostCSS is used by Google, Twitter, Alibaba, and Shopify. Its most popular plugin, Autoprefixer, is one of the most universally praised CSS processors available.
PostCSS can do the same work as preprocessors like Sass, Less, and Stylus. But PostCSS is modular, 4-40x faster, and much more powerful.