Comment by nightpool

Why use a new custom-built protocol instead of adding support for the Web Push protocol already supported by Firefox, Chrome, Safari and others? https://web.dev/push-notifications-web-push-protocol/. Now backend developers have Yet Another Push Notifications Protocol to build support for, instead of being able to just easily plug into their existing code. As a benefit, since all notifications through the Web Push protocol are end-to-end encrypted, there's no concern about "trusting" the distributing server like there seems to be in this protocol (based on https://unifiedpush.org/spec/server/)