I trained myself out of using backspace for back precisely because of the number of times I'd end up in an input box and it would stop working. I use something like cmd-[ now, I think. I guess my point about space for pagedown is that it just seems unnecessary - I wonder how many keyboards don't have a PgDn key. Overloading printable characters, when non-printable alternatives exist, seems like a mistake.
Most laptop keyboards don’t have a Page Down key, though most that don’t will have Fn+Down emit Page Down. Space and Shift+Space are super useful on laptops especially.
