HDR on YouTube -- or anywhere for that matter -- is table stakes these days, and I don't think it's silly to say so. Would you buy a television that can't handle HDR in 2022? A phone that can't handle HDR in 2022?
Two points:
1. There used to be a time when Firefox was way behind, and their big "come back push" marketing argument was that they had essentially 100% compatibility on something like the top 1 million web sites. Now? Major features are missing on the number one video streaming site. That's not a small regression!
2. Browsers are essentially operating systems now. Applications are written for web browsers, not for Linux, Mac, or Windows. There's a reason even Microsoft is using Electron for most of their new applications. Essentially we now have Google-Web-OS and Apple-Web-OS, and even Microsoft has failed to deliver their own Web-OS (Edge). Mozilla's attempt is like the Linux Desktop compared to MacOS. It fails on the basics in the same way Linux can't get sound right or HDR video playback working. Meanwhile MacOS (and Safari!) has calibrated wide-gamut colour that Just Works.
We only interact with computers through two senses: sight and hearing. Mozilla can't get one of those two working consistently, correctly, or with the full capabilities of displays available for a decade. (I bet Firefox has languished in its sound capabilities also, but I'm not an audiophile, so I don't know the specific issues.)
I have a HDR laptop, a HDR TV, and a HDR phone. All three run browsers. Firefox is missing this key feature on all three platforms, making it a no-go for manufacturers, or anyone who wants to use the #1 video streaming site in the world... with their web browser.
This is my point: Mozilla has wasted time on vanity projects while they fired to Servo team.
The top comment on a similar post 10 months ago agrees with me: https://news.ycombinator.com/item?id=28813250
Maybe that commenter is irrational too?