Oh, it definitely happens. Especially when you have certain multinational corporations with their own browser and far too much market share who claim their onli

Oh, it definitely happens. Especially when you have certain multinational corporations with their own browser and far too much market share who claim their online office suite couldn't possibly work perfectly on <insert competitor>'s browser because it isn't as shiny as their browser. The only other exception I've seen recently is because of genuinely lacking features e.g. WebMidi, or where a really small outfit just doesn't have the time to ensure 100% compatibility with every browser yet, so puts it out there with heavy warnings. But IME, it is vanishingly rare to find a website that says "Nope, can't do this with firefox" and flat-out refuse to serve you without a very good reason, and when they do, they're idiots if they use the highly-spoofable UA to do that check. (But I would love to see counter-examples - as I say, I'm sure they exist, but outside the corporate world, I haven't seen it in a while)