Wow, what the hell was that javascript tab change ? Not sure if I hated or loved it, but that was the first time I have seen that.
Favicon can be part of the DOM ever since icons via ~~meta~~ link tags were introduced, because they can be more hi-res than the default favicon.ico file.
I've never thought about the ability to swap it dynamically though, although I've seen some site do that before.
https://developer.mozilla.org/en-US/docs/Web/API/Document/vi...
https://developer.mozilla.org/en-US/docs/Web/API/Document/ti...