That's not a problem with the whole concept, it's just a problem with how they did it. Volume controls are built in to the native music player element
Yep, that's right. The "mini player" on Windows has exactly the same problem. Its a big box of 200x100~ pixels or so and it doesnt include a volume control, you have to exit the mini player. I do not understand the reasoning of such decisions, really. It maybe looks cute but It's pretty useless.
Shouldn't that be handled by the native platform?
I have a thinkpad from 2008 with dedicated volume keys...
No, because I could be playing a game with its own volume, while also being in a webrtc call with someone on another tab. Volume control for a music player is no-brainer.
Not so long ago I had this problem on bandcamp too, but they do not attempt to run without JS at all. I regulated volume for the website in my OS instead. However, when it switches to the next track, it would blast off my ears again.
This issue is not limited to websites like the one linked here.