In general all of the simple dialogs (alert(), prompt(), confirm(), and beforeunload) are deprecated and being removed slowly but surely from the web platform. They use trusted browser UI, which opens them up to abuse, and they block the event loop, which is not in keeping with the web's cooperative task model. So this is just a first step toward eventually completely removing them, which will take many years.

https://github.com/whatwg/html/issues/6897#issuecomment-8857...