Play a drawbar synth organ in your browser with a MIDI controller

Note that the "programmable synth" part of this still works even without Web MIDI, but having a MIDI controller with built in sequencer drive the whole thing is way more fun =)

It's also more practical because the in-browser arranger is constrained by whether or not JS gets enough time to run the BMP ticker "smoothly", which your MIDI controller is unlikely to have a problem with: if you can sequence/arp with it, it has one built into the hardware and all it needs to do is send midi control codes to the browser to act on.