Airhorner was the first canonical ‘Progressive Web App’. Whilst it is a toy, it served to show how you can build a simple Service Worker to make your experience work offline.
Because of it’s simple nature it was used by the Chrome team and many publications as a reference for the core components of PWA: Offline and installability. For example, it was the first application to use ‘splash_screen’ and was used to find issues with the way we generate icons. It was also used as the application that demonstated ‘Web APK’ and what it means to be integrated in to the user’s device like an Android App is.