Backspace to go back, OTOH, I was aware of, but I doubt I ever wanted. Alt-Left is easier to use because it's less modal. Still bothers me in general when devs screw with established defaults.
I do find the spacebar behavior isn't super predictable when I am trying to pause things, such as youtube .
I've never managed to consistently replicate it by having had the player in/out of focus already, not/scrolled, etc. odd.
EDIT: and before anyone points out that if I'm already using extensions I could easily add another, yes and I would if I had to, but it's not ideal, because 1. Extensions crash, fail to load, or aren't allowed by internal browser pages, and 2. You sometimes have to use other peoples' machines and it's annoying when basic functionality is locked away.