JavaScript is disabled
Plug in your MIDI keyboard.
Refresh this page.
Pick the keys and notes you want to practice.
Click the button.
You may need to enable MIDI support in your browser.
In Firefox you need to go to the about:config page
and set dom.webmidi.enabled to true.
If you have your DAW opened it can prevent the browser from receiving MIDI events.
VexFlow - an open-source library for rendering music notation
webaudio-tinysynth - a small synthesizer written in JavaScript with GM like timbre map