That's really cool. I had no idea Web MIDI API existed to allow stuff like that (though it appears to still be Chrome-specific :/ ).
I also made this synth that uses it, you can use your typing keyboard too: http://www.errozero.co.uk/stuff/poly/
...and a little pong game, you can control the paddle with any knob on a connected MIDI controller. http://errozero.co.uk/midi-pong/