A client calls you one day. He says he cannot access your web site. So you hurry up to check what's wrong. Everything seems perfectly normal. The problem is that the client is using Internet Explorer 5. Kaboom! So what's next? Internet Explorer 5 has an infamous and poor support to web standards, so even the most simple things get complicated in this browser, not to say impossible to do. The most obvious thing to do is to convince your client to upgrade his browser. Here another problem: he should upgrade his operating system as well. Is there a solution?
By the way, the call was real, not just an example. sad