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