Matrix is also a website and you don't need an app to use it. The first time I used Matrix, I didn't use an app, I merely signed in on a browser window (in my case, Mozilla's instance). I first signed up on my work laptop, then later signed in on my desktop and had to confirm the new account on my laptop before my desktop would work with the same account.

The more devices it's on the better, but it's totally usable with just one web client.

If Matrix can do that, lemmy can as well. It would probably degrade the user experience because you'd need a decryption step for every post and comment you load (just like loading a new Matrix room), but it is technically possible.

I'm not necessarily asking for every comment to be encrypted, I just think it would be a good idea for DMs to be encrypted using keys the admin doesn't have access to. It would be cool for communities to allow encryption as an option as well (i.e. all posts and comments would be E2E encrypted to all members, and not viewable unless you join), but it shouldn't be the default everywhere.