This is a case where the solution you’re coming up with, which has never been the normal solution on the web but people definitely try it from time to time (perhaps because it is how you’d implement such a thing in almost every non-web GUI toolkit), causes certain very specific problems which render it unsuitable for general web content, particularly making life harder for quite a lot of desktop users (where Space/Shift+Space for scrolling is very common).
This is not a case of “well akshully” correctness, where the difference is immaterial. It’s a specific, concrete thing where it’s good to learn the behaviour of the platform, so that you work with it rather than against it, and not cause unnecessary difficulties to normal users.