PostCSS is a tool for transforming CSS with JavaScript. It parses a stylesheet into an abstract syntax tree, lets plugins inspect and modify rules, declarations and at-rules, and then stringifies the result back into CSS, which makes it the foundation for a large ecosystem of focused styling tools rather than a single monolithic preprocessor.
The official site documents installation, the plugin API, how to run it from the command line or through build tools such as webpack, Vite, Rollup and Gulp, and lists widely used plugins including Autoprefixer, postcss-nested, cssnano and postcss-preset-env, along with guidance for writing and publishing your own plugin.
It suits front-end teams that want to combine exactly the transforms their project needs, framework maintainers building a styling layer, and organizations that must support legacy browsers while still using modern CSS syntax, as well as anyone automating repetitive stylesheet rewrites across many files.
Compared with Sass it adds no new language by default and can be mixed with native CSS features; compared with a full CSS framework it controls the build pipeline rather than prescribing class names; the trade-off is that a useful setup usually requires choosing and ordering several plugins yourself.
List plugins explicitly and pin their versions because order changes the final output, keep browserslist configuration in one place so Autoprefixer and Babel agree, and run the processor before minification so generated vendor prefixes are not stripped or duplicated by later steps.