"So many people have the attitude 'if it works in what I use then that is good enough'. That is a bad attitude to have. There are so many people who don't or can't use what you use. They should not be exluded (see my Site Design tutorial for a more detailed reason).
I notice that most people who have that incredibly arrogant attitude are using an IBM (compatible) PC with a nice, fast processor running Windows 2000 or above, with a nice fast Internet connection. Generally they use Internet Explorer or occasionally one of the Mozilla browsers. People who use anything else usually have suffered at the hands of those bad designers and will usually have a better attitude as a result. The thing that these designers forget is that the idea of a Web page is to provide information or services to other people; different people. Demanding that they use only the "correct" programs or setup simply means that people who do not have them will just go elsewhere. They will not give in to unreasonable demands, they will just find something else to look at."