.postcssrc.js

module.exports = {
  plugins: [
    require('cssnano')({
      preset: [
        'advanced',
        {
          autoprefixer: { add: true },
          discardComments: { removeAll: true },
          discardDuplicates: true,
          discardEmpty: true,
          discardOverridden: true,
          discardUnused: true,
          zindex: false
        }
      ]
    })
  ]
};