[ksherlock]

That URL also explains why it only works on chrome.

[kzakka]

Hi author here! The app should work on mobile/desktop and was tested on both Safari and Chrome. I've heard it's buggy for some people (unclear if it's an older hardware problem) but you can try this embedded demo which works better: https://kzakka.com/robopianist/#demo

[squarefoot]

It works here on Firefox, although playing is sorta syncopated. Very likely a timing issue.