Ironically, YouTube is now forced to support a browser that has terrible standards support, entirely of their own making: Cobalt[1].
YouTube on TVs is actually a web app that loads into a stripped down, custom webview. The YouTube team doesn't have the resources to implement many web APIs, so they implemented just what they needed.
The problem is that they can't reliably update Cobalt versions on TVs, they can't ask users to update, and they can't just break older TVs in the wild. So the YouTube on TV frontend (not YouTube TV the service) has to only use APIs they shipped like 10 years ago.
And because it takes so long for an old Cobalt version to go out of support, they don't invest in implementing new features because they wouldn't be usable anytime soon. 10 years ago I was in a meeting with them where they said they couldn't implement something because they wouldn't be able to use it for 5 years... They still haven't implemented it.
YouTube on TVs is actually a web app that loads into a stripped down, custom webview. The YouTube team doesn't have the resources to implement many web APIs, so they implemented just what they needed.
The problem is that they can't reliably update Cobalt versions on TVs, they can't ask users to update, and they can't just break older TVs in the wild. So the YouTube on TV frontend (not YouTube TV the service) has to only use APIs they shipped like 10 years ago.
And because it takes so long for an old Cobalt version to go out of support, they don't invest in implementing new features because they wouldn't be usable anytime soon. 10 years ago I was in a meeting with them where they said they couldn't implement something because they wouldn't be able to use it for 5 years... They still haven't implemented it.
[1]: https://developers.google.com/youtube/cobalt