Adding PostCSS Plugins

PostCSS may be used standalone, or even as a secondary step after your core Sass/Less compilation. Either way, the end result will be the same. Think of PostCSS plugins as layers of an onion, so to speak. Each layer has the chance to operate upon your stylesheet, and modify it in some way. Once complete, those styles are then passed on to the next layer of the onion. Let’s review this workflow, when using Laravel Mix.
Source: Laracasts