PostCSS is a tool for transforming styles (CSS) with JS plugins. It allows you to write nested CSS code that is readable and maintainable.
It supports the ability to have mixins, which can be used to easily and clearly define style behavior for responsive sites.
It also allows you to use latest CSS standard without the fear of not working in current or old browsers.
Used with Autoprefixer – which automatically prefixes CSS code for old Browser specific compatibility.