PostCSS is a tool for transforming CSS with JS plugins. These plugins can support variables and mixins, transpile future CSS syntax, inline images, and more.
PostCSS is used by Google, Twitter, Alibaba, and Shopify. Its plugin, Autoprefixer, is one of the most popular CSS processors.
PostCSS can do the same work as preprocessors like Sass, Less, and Stylus. But PostCSS is modular, 3-30 times faster, and much more powerful.
PostCSS itself is very small. It includes only a CSS parser, a CSS node tree API, a source map generator, and a node tree stringifier.
All CSS transformations are made by plugins. And these plugins are just small plain JS functions, which receive a CSS node tree, transform it, and return a modified tree.