[iggldiggl]

> They do it on mobile only as well, videos do play in the background when I use the desktop site.

Or if you use a browser that supports add-ons and install something like https://addons.mozilla.org/android/addon/video-background-pl... to block Youtube from accessing the APIs it uses to detect when a tab has been backgrounded.

And the reasons aren't that unknown - if I'm not mistaken background playback is one of the selling points of a Youtube Premium (formerly Red) subscription.