Un petit problème à signaler, avec les raccourcis clavier + et -, la tonalité change, mais aussi le tempo :-/
Sinon, ça marche plutôt bien

A great Firefox alternative to the Chrome-only https://transpose.video, which does what it says on the box.

The pitch-shifting algorithm does produce slight artifacting at certain pitches, but overall works well enough to be able to sing along.

*However, keep in mind that for it to work well, the (preserved pitch) option has to be turned off.