It is normal practice for each browser to have its own user-agent, no? But the fact that Google intentionally detected it and used polyfills or straight up invalid JS at the time was insane. A similar spin today is "Your browser is unsupported" you see here and there. When a major platform such as YouTube does it, it is really impactful.

It would never do feature detection, would give lower quality h264 video, etc. Back then, there was really nice third-party application myTube which had made this less of an issue but it was eventually killed through API changes.