Installed platform apps can be launched even when offline. In this situation they often include a page letting the user know internet access is not available. Some developers even create a full set of features that can be used while offline.
With web apps, the capability to work offline is more recent; it became possible with the launch of the Service Worker API. Also, when websites became installable, PWA added a requirement to register a service worker and implement its fetch() method in order for a web app to be installable.
The idea was to incentivize developers to provide their users at least a notice that the app couldn’t be used at the moment.
Now developers won’t need to do anything to get a default offline page. From Chrome 109 on Android, the browser will automatically generate a page that lets the user know when they are offline.
If the app doesn’t implement a custom offline experience, when the user is offline, the default offline page uses the app’s icon and simple You are offline message. Like the example below.