Some _can_ use IE/FF/Safari, but when they come across ajax-heavy webapps it's like "what the?!". Plus it is entirely possible to make a fairly accessible shiny web2.0 webapp (e.g. gmail even without the simpler html mode and google reader with aria support)...although they are generally in the minority.
A very, very small minority are going to have a browser with no javascript support. It's important to have some sort of fallback, but I guess it also depends on your audience. No point in spending that time if all you're doing is catering to teens to advertise something, for example.
A very, very small minority are going to have a browser with no javascript support. It's important to have some sort of fallback, but I guess it also depends on your audience. No point in spending that time if all you're doing is catering to teens to advertise something, for example.