I wouldn't be defending them if YouTube refused to work with a "strange" user agent, but that's not what happened. Judging by the screenshots, YouTube still worked, it just refused to use a new design. The old design is still perfectly functional.
Chromium-based Edge is not stable. It's a new thing that I don't expect website owners to test against. The error message showed that the new design is tested against the latest version of Edge. Complete rewrite of Edge still hasn't replaced the old Edge.
Also, no, the web is not a standard. There's no fixed set of things that a browser should implement and call it a day. It's constantly-evolving depending on the needs of the owners of the website. It's why nobody dares to create a browser from scratch nowadays.
Chromium-based Edge is not stable. It's a new thing that I don't expect website owners to test against. The error message showed that the new design is tested against the latest version of Edge. Complete rewrite of Edge still hasn't replaced the old Edge.
Also, no, the web is not a standard. There's no fixed set of things that a browser should implement and call it a day. It's constantly-evolving depending on the needs of the owners of the website. It's why nobody dares to create a browser from scratch nowadays.