If you use keyCode instead of charCode I think you'll get the same result on every keyboard layout, but of course if you tell people to use C, it means the key where C is on a US keyboard, not really the key where a C is printed. I believe Dvorak users are conscious enough to understand that though.

Arrow keys, space, control, shift etc. are all normal, but I guess how capturable those are from JS varies. Making them configurable?

Qwerty X maps to Dvorak Q; Qwerty C maps to Dvorak J. That is, listening to Q and J when the user is using Dvorak, as X and C on a Qwerty keyboard.

its pretty capturable. Ok the idea to configure keys make a lot of sense!

I would have made space shoot, too, incidentally (arrows + space should be the same for just about every keyboard out there), but configuration itself is a great idea.

I also wish that the length of time you held down the jump key controlled the height of your jump rather (up to a maximum, obviously), rather than one-size-fits-all jumps, but that's minor.

Play an old NES game that used that system like Super Mario Bros. (any of them) then play your game to see how much more in control you feel.