Firefox added split view where you can look at two (or more) webpages side by side. This is a lifesaver when you have to fill up a form looking up stuff from another page!
Chrome does this split-screen. Web browsers are operating systems, for all intents & purposes.
Ask any Emacs evangelist.
Isn’t this kind of the job of the OS windowing system? It’s maybe slightly nicer to share the window chrome for two tabs but it’s not like looking at two browser tabs in parallel was impossible before.