I mentioned Space as it’s the one I’m used to, but now I think back on it, all the other navigation keys get messed up inside a text box too (Up, Down, Left, Right, Home, End, Page Up, Page Down), so it’s not actually just about Space.
Dunno why you’d consider Space for page navigation anachronistic. Space has always been a flexible “do all kinds of different things as useful” key, and Space/Shift+Space scrolling in focusable caretless content areas has always been popular and universally supported. But if we were talking about troublesome old key behaviours, I’d be with you on Backspace to go back being bad, which some browsers still do. (And in file managers, it goes up a directory, which is a useful operation, but you can use Alt+Up for that too. All up, if it weren’t for muscle memory, I’d say nuke non-deletionist Backspace behaviour, but muscle memory is a serious thing, so we’re stuck with Backspace being weird like this.)
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.
Dunno why you’d consider Space for page navigation anachronistic. Space has always been a flexible “do all kinds of different things as useful” key, and Space/Shift+Space scrolling in focusable caretless content areas has always been popular and universally supported. But if we were talking about troublesome old key behaviours, I’d be with you on Backspace to go back being bad, which some browsers still do. (And in file managers, it goes up a directory, which is a useful operation, but you can use Alt+Up for that too. All up, if it weren’t for muscle memory, I’d say nuke non-deletionist Backspace behaviour, but muscle memory is a serious thing, so we’re stuck with Backspace being weird like this.)
The php.net front page should certainly not have autofocus, though perhaps https://www.php.net/search.php could.