The end-to-end encryption also works on the web. I’ve used it and it’s excellent. You need to use a PIN to access your past messages from their backup HSMs, but other than that it’s completely transparent.
If I understand the parent comment right, this was an argument against ProtonMail's End-to-End Encrypted Webmail 5+ years ago.
The argument being that some assurances typically associated with E2EE (that "even we can't see what you're doing") are shakier without a disinterested third party serving the application to the user. If you have some target user `Mr. X`, and you operate the distribution of your app `Y`, you could theoretically serve them a malicious app that sidesteps E2EE. And since it's just a web app: the blast radius is much smaller than if you were to go through the whole update process with Google or Apple and have it distributed to all users.
