Post CSS is a tool used to transform styles with the help of JS plugins.
When you write the CSS for a website, you don't really have the ability to use optimised and modern features, and the user wants a fast, functional website that supports legacy browsers.
Post CSS takes a CSS file and converts it into an abstract syntax tree, from which API's written in JS can be used produce a desired output.
It also handles pre-processing similar to frameworks like SASS and SCSS.
For example- the Preset ENV plugin can convert your modern CSS while post CSS adds the polyfills needed to make your code compatible to the browser you use.
You could use a color API to find all the color declarations in your code which you can then use as you feel.
Post CSS gets more npm downloads than Typescript and Webpack combined.
Next JS uses Post CSS under the for a streamlined experience. Wikipedia uses Post CSS for their website.
Post CSS can altogether replace multiple frameworks or can be made to work with existing ones.
Highly recommend you give this a look. You've mostly used post CSS without even realising it!