Visualiser
Like the CD player on an old console: put some music on and watch it. Each scene is a bit of maths (tunnels, oscilloscopes, polar curves, wireframes, Fourier epicycles, a Lorenz attractor, an orbiting sun) pushed around by the bass, mids and highs.
Pick something to listen to. The built-in track is an original synth loop made live in your browser.
The numbers
Listening to other music
Your music file plays a song from your computer; it never leaves your device. Tab or device sound listens to a browser tab (pick it and tick Share tab audio) or, on a computer, everything your device plays, including the Spotify app: choose Entire screen and tick Share system audio. This works in Chrome and Edge on Windows, and on recent Macs. Phones and tablets don't let websites hear other apps, so there use Microphone, which listens to music playing out loud. The browser asks first, and nothing is recorded or sent anywhere.