This is amazing! A couple things:

1. Doesn't work on Firefox because of compatibility. (Web MIDI could not be enabled: DOMException: WebMIDI requires a site permission add-on to activate index-C82-1cwq.js:10:56869 doInitialize https://lend-me-your-ears.specr.net/assets/index-C82-1cwq.js... ) . Might be worth detecting browser and telling users straight up before they waste any time.

2. How did you come up with the melodies? Hand crafted? AI? Asking because I plan on building this for Gitori(www.gitori.com) at one point and my original idea was taking snippets from MIDI files of famous classical pieces/jazz solos

Also, TIL about the Simon game. Might buy this for my 6 year old :)