some sites assume you don't use your web browser. for today i don't take issue with performance (i mean i do but this post is not about this), but with the use of "spa" architecture.
on web pages we have this neat concept called "links". you can click them, and then you "follow" them. but, you can also use middle mouse button to follow them in a new tab.
frontend developers sometimes don't bother with supporting that behavior. for example, if you are viewing a channel on youtube you can't open "videos" "tab" in another browser tab. which annoyed me more than once (i'm a heavy tabs user).
something even more egregious is not allowing more than one instance of a site. it maybe makes sense in the context of banking apps, but there is literally no reason to not allow that on a certain polish train ticket selling site.
in the case of youtube let's hope it doesn't spread to recommended videos.