Dave Hyatt, the developer who worked on browsers such as Mozilla, Firebird, Camino, and Safari, tries to describe the difficulties an online application project has to deal with.

This is very similar to what my project is going through right now. The client’s QA group is performing browsers/OSes compatibility testings and the sort of things our web application has to cope with are phenomenal! In fact, the actual permutations work out to something in the thousands, and the client QA is running around 10,000 test scripts. Each combination presents some slightly different behavior, especially regarding JavaScripts. The most trouble we are having is with Netscape 4.7x and WinME, though not necessarily in that combination.

It does not help that we are using ASP.NET to produce DHTML to implement a page design that is cool but not necessarily backward compatible with older browsers (i.e. NS4.7x).