For those, there's this: https://butterchurnviz.com

It's a WebGL port of the Winamp visualizer.

Back then I could run a music visualizer offline. I have no idea why this website requires me to log in to Spotify.

It doesn't sync to live audio it records/grabs. It uses the Spotify API that provides rythmn/intensity mappings for most (all?) songs. So basically the web page asks the API which song you're currently playing on Spotify and what BPM it has and it renders the visualization accordingly.

Also:

> In the future I'm going to tap into Spotify listening history so I can surface music recommendations and other features.

See: https://github.com/zachwinter/kaleidosync/blob/6136f79988867...

I'm running one on my TV right now through Kodi