I tried this in Firefox and the buttons didn’t seem to be working. Upon inspecting the page with the developer console, I noticed this error:
Uncaught (in promise) DOMException: WebMIDI requires a site permission add-on to activate
I have webMIDI enabled in my about:config so I don’t know what could be the issue. Maybe it’s my ad blocker.
Anyway it would be cool if the web app could catch this error and give the user instructions on how to work around it (e.g. change their browser settings, switch to a supported browser, etc). Judging from the screenshots, the tool looks really cool and I wish I could use it.