PostCSS is a tool to transform CSS styles using JavaScript plugins. Popular plugins include autoprefixer, future CSS transpiling, CSS linting and media queries.

Autoprefixer helps developer to automatically generate vendor specific prefixes. Take a look at the interactive demo of Autoprefixer in action

POSTCSS-cssnext is a PostCSS transpiler plugin that transforms new CSS specs into CSS that is more compatible with browsers. This includes CSS nesting, custom selectors and media queries.