W3C Priority 3

If you are unfamiliar with accessibility issues pertaining to Web page design, consider that many users and particularly in this sector may be operating in contexts very different from your own:

• They may not be able to see, hear, move, or may not be able to process some types of information easily or at all.
• They may have difficulty reading or comprehending text.
• They may not have or be able to use a keyboard or mouse.
• They may have a text-only screen, a small screen, or a slow Internet connection.
• They may not speak or understand fluently the language in which the document is written.
• They may be in a situation where their eyes, ears, or hands are busy or interfered with (e.g., driving to work, working in a loud environment, etc.).
• They may have an early version of a browser, a different browser entirely, a voice browser, or a different operating system.

For a more detailed explanation please visit: W3C's full checklist