webpack.config.js

const postcssPresetEnv = require("postcss-preset-env");
const postcssImport = require("postcss-import");
const cssnano = require("cssnano");
const MiniCssExtractPlugin = require("mini-css-extract-plugin");

module.exports = (env, argv) => {
  return {
    module: {
      rules: [
        {
          test: /\.js$/,
          exclude: /node_modules/,
          use: {
            loader: "babel-loader"
          }
        },
        {
          test: /\.css$/,
          use: [
            MiniCssExtractPlugin.loader,
            { loader: "css-loader", options: { importLoaders: 1 } },
            {
              loader: "postcss-loader",
              options: {
                ident: "postcss",
                plugins: () => {
                  const plugins = [
                    postcssImport(),
                    postcssPresetEnv({
                      stage: 0
                    })
                  ]

                  if (argv.mode === 'production') {
                    plugins.push(cssnano());
                  }

                  return plugins;
                }
              }
            }
          ]
        }
      ]
    },
    plugins: [
      new MiniCssExtractPlugin({
        filename: "main.css"
      })
    ]
  };
};