As a final note in our discussion of perception, it should be emphasised that not all of our users have the same perceptual capabilities.

Graphical user interfaces are – not surprisingly, given the name – highly visual in the demands they make on users. Much of the user interface technology that is being designed for personal computers and used on the world wide web makes strong assumptions about the perceptual capabilities of users, and designers are therefore often excluding sections of the population whose vision, hearing, or other senses and abilities are impaired. Aside from any moral considerations about making technology accessible to all, there are good business reasons for considering access to our designs: the more people who are unable to use out designs, the more potential customers we have lost. This is especially true on the web, where a large number of potential customers will look at a site, and decide whether to continue to use it on the basis of whether its design helps them to carry out their tasks easily.

Sometimes, designing for inclusiveness and accessibility isn't only an ethical consideration or a matter of commercial good sense. In some cases it is also a legal requirement. For example, in the United States, the Americans with Disabilities Act (ADA) requires that certain organisations and bodies (particularly governmental bodies) provide "effective communication" when they use the Internet to make information and services available. Specifically, this ruling notes that web sites must be accessible to those using screen readers and receiving web information through auditory rather than (or in addition to) their visual sense.