Описание тега postcss-loader

NoneConfigure postcss for css assets transformations inside webpack process

Instead of postcss.config.js postcss configuration can be set inside webpack.config:

{
  loader: 'postcss-loader',
  options: {
    config: {
      ...
    }
  }
}

Other problematic that cover this topic: cascade configuration and postcss plugins configuration.

https://github.com/postcss/postcss-loader