Original comment author here -- we can talk about using user agents to detect edge-case bugs as something that's valid to do. We can also talk about whitelistin

Original comment author here -- we can talk about using user agents to detect edge-case bugs as something that's valid to do. We can also talk about whitelisting specific user agents that don't require polyfills so you can speed up initial page load. I think these are reasonable things to suggest. So you're correct that there is some room for nuance and you're right to point that nuance out. But Youtube isn't doing either of those things -- it's assuming that a user agent it doesn't recognize by default isn't supported, and rather than serve a feature-detecting version of the page, it's serving users a message to use a different browser. I still maintain that this is basically never the right thing to do (or at least it's so close to "never" that doing so should require some very good, app-specific justifications). People are debating whether or not you can use JS to feature-detect every single specific bug, and whether or not you need to go the long path on feature-detection and do everything up front, or whether you can skip some steps for performance optimization, and they're missing the entire point of progressive enhancement. If you can detect Chrome or IE 6 and speed things up or avoid a bug, fine. But it's bad practice to rely on that detection. It's bad practice to have a single list of browsers you support and to turn everything off everywhere else. Partially because it's bad for the web in general, and partially because from a more practical perspective it's not always safe to assume that just because you see a user agent every feature you want will be supported -- extensions exist, custom-compiled browsers exist, and sometimes user agents are just reported wrong for fringe browsers. It's also just plain bad for the user because falling back on feature detection and adding a few seconds to page load is nearly always preferable to being told to download and install a new program. My original comment wasn't that user agent strings should be universally ignored, it was that it's unfortunate to see a company as respected as Google so fundamentally misunderstand something that's been widely taught as best-practice on the web for over a decade. I kind of feel like some of the people objecting are missing the forest for the trees -- talking about whether there's ever any valid reason at all to ever try to detect a specific browser instead of the underlying idea that the web uses a living standard and that it was designed to be client agnostic.