To obtain a credential, ask for a QR code of the driver record to be displayed, and scan it with you mobile phone. That will result in a credential being issued to the default browser of your phone.
You may present the credential by scanning a QR code or over the web by clicking on a login button. (Presentation over the web is not implemented yet.)
A service worker is created automatically when the credential is issued. For debugging purposes, you may also use this link to register the service worker after it has been modified.