Alert is pure garbage and should not have made it past the 90s. Also, basic auth popups need to go too. Not sure why browsers would ever make those focus stealing in the first place. There should not be one single way for a web application to steal focus. The current workaround is to download a buggy ad blocker (last time I used chrome, just like firefox it has no way to turn popups off).

Edit: I just remembered alert no longer steals focus on modern browsers (IIRC). But basic auth still does (at least on my 50 year old fork of firefox).