PostCSS is a great tool for enhancing CSS and it has an extensive ecosystem - postcss.parts has a great catalog of available plugins. It also make it quite easy to write your own plugins to extend CSS in weird and wonderful ways. If your plugin is particularly lightweight though, you might not want to bother with the overhead of creating a package, publishing it to NPM etc. Fortunately, it's quite easy to load a PostCSS local plugin inline.
