There's one area of an application that is traditionally difficult to test, and that's the UI. This is particularly the case for Graphical User Interfaces (GUIs), as used in web applications. One reason for this is that users typically have a lot of flexibility in how they display their interface, and this is especially true for web browsers. A naive pixel-based testing approach is going to be extremely fragile.
You need to design your application to be easily testable, and the UI is no exception. While it is possible to test the UI of a poorly-designed legacy application, and this may even be the easiest testing approach, if you consider UI testing upfront, then life will be a lot easier. For example, including sensible ...
