create-react-app-webpack-shadow-dom.js

// create-react-app v2.1.8, webpack.config.js, line 69
const getStyleLoaders = (cssOptions, preProcessor) => {
  const loaders = [
    isEnvDevelopment && {
      loader: require.resolve('style-loader'),
      options: {
        insertInto: () => document.getElementById('root').shadowRoot,
      }
    },
    // ...
  ].filter(Boolean);
  // ...
};