git clone
npm install
npm start

Notice how the iframe says "Not working :(" and the console shows errors like these (Firefox):

GET http://localhost:8080/custom-assets/c6173703b2eddc336511.js NS_BINDING_ABORTED
A resource is blocked by OpaqueResponseBlocking, please check browser console for details. c6173703b2eddc336511.js
Loading failed for the <script> with source “http://localhost:8080/custom-assets/c6173703b2eddc336511.js”.

Update the webpack config to uncomment the allowedHosts, and then restart to see the iframe say "It works :)"