Many users have difficulty with detailed mouse movements and may also have problems holding down multiple keyboard keys simultaneously. Most of these issues should be taken care of by improved browser design and should not concern content designers except for the advice not to design imagemaps that require extremely precise mouse positioning. By cognitive disabilities, I do not refer to below-average intelligence, though many sites will have to support such users as broader segments of the population go online. Currently, the Internet is so difficult to use that users need to be fairly intelligent to figure it out, but even these smart users sometimes have cognitive disabilities. Unfortunately, cognitive disabilities have not been the focus of as much user interface research as physical disabilities, so I only have a few examples.
People vary in their spatial reasoning skills and in their short-term memory capacity.
Programmers and graphic designers tend to get uncommonly high scores on tests of spatial reasoning skills and are therefore good at visualizing the structure of a Web site. It is safe to assume that most users will have significantly greater difficulty navigating a Web site than its designers have. Simplified navigation helps all users, but is a required enabler for users at the opposite extreme of the scales. Users with dyslexia may have problems reading long pages and will be helped if the design facilitates scanability by proper use of headings as noted above. Most search user interfaces require the user to type in keywords as search terms.