postcss load plugin

cmd.bash

npm i -g postcss-cli
postcss in.css -o out.css

in.css

body {
  h1 {
    color: red;
  }
}

package.json

{
  "name": "xxx",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "postcss": {
    "plugins": {
      "postcss-nested": {}
    }
  },
  "devDependencies": {
    "postcss-nested": "^1.0.1"
  }
}