add a cacheable offline page

[okjintao]

Progressive Web App

Page does not respond with a HTTP 200 OK status code when offline. The Progressive Web App standard suggests that Websites should provide a custom offline page that works as much as possible as their online versions.

Potential improvements

Add a service worker to the App and use it to cache files locally. When offline, use the service worker to return locally cached versions of the features that don't require a network connection.

[okjintao]

https://www.html5rocks.com/en/tutorials/appcache/beginner/

https://web.dev/offline-fallback-page/