Very nice work! There seem to be some issues for me with the lower strings on guitar: the low E registers as a B, for example. Higher strings seem to work well though. Not totally sure what's going on, but could probably be attributed to mic quality :)

Edit: after more playing around (heh) it does start to pick up the almost-E notes, F, G, etc. but is a little inconsistent. I tried monitoring the mic and even recording to play back what got picked up, and it sounds just as clear as the higher notes. Maybe something numerical happening in the Javascript fft?