[chrisseaton]

> can anyone explain to me why Signal can't be implemented as a completely offline web-app

It's a messaging app. What's the point of a messaging app if you're never connected to anyone else to message them?

[hardwaresofton]

Edited for clarity, what I meant there was offline-first/PWA. The idea of being able to do things completely offline and use some other medium is also interesting, but what I mean to convey is web apps built with the expectation that the internet may not be connected (and keep working).

Generally offline-first/PWAs can be made "installable" by setting metadata on the page that hosts them and doing other things.