IMO this is less a fault of websites that contain media, and more the fault of the OS that mediates what media is playing.

In other words, there should be a toggle to "default mute the web and keep spotify running".

I disagree. How is the OS supposed to know the difference?

If you want to make a nice animation, how about not including an audio track? Or using the attributes to specify it should be played muted so it won’t cut my audio by default (what YouTube and others do).

The OS (or browser, rather) probably shouldn't allow autoplaying sounds at all.

What is it that's only allowing you to play audio from a single source at a time? On my desktop that sound just plays over other sounds, nothing pauses. I hate auto-play media as much as the next guy but your real problem is an awkward configuration or oddly limited device.

I’m not on a desktop.