It sounds like a good idea, but every time I've seen this implemented it's been as a simple whitelist lookup against user agent. This has two problems. First, user agent is... unreliable. Second, people that use less common systems are erroneously told the page won't work. (I've lost count of the number of times a website has told me that my operating system isn't supported yet the page works fine. What does my os have to do with a web page anyway?)