Accessibility

Accessibility is the "ability to access" web resources. Usability, by contrast, describes how web resources can be used to accomplish certain tasks by certain users. If you can't access web resources, you can't use web resources.

The W3C issued a working draft on December 11, 2007 Web content accessibility guidelines 2.0.

Web Content Accessibility Guidelines 2.0 Principles

Perceivable - Information and user interface components must be presentable to users in ways they can perceive

Note the special problem of CAPTCHAs. These are texts that require a human being to read distorted text and then correctly input the text. In this manner, computers can distinguish human agents from digital agents.

The guidelines suggest providing a text alternative.

Provide text alternatives
Provide synchronized alternatives for synchronized media
Content can be presented different layouts
Foreground and background are easily distinguishable

 
Techniques to ensure foreground and background colors can be selected by the user 
      Specifying foreground and background colors in CSS OR
      Providing a color selection tool that allows a pastel background  OR
      Providing a multi color selection tool on the page for foreground 
         and background colors (JavaScript, Future Link) OR
      Using a technology that has commonly-available user agents that can change 
         the foreground and background of blocks of text

Operable - User interface components and navigation must be operable

Keyboard accessible
Sufficient time to respond
Does not flash more than three times in any one second
Navigatible, findable, oriented

Understandable - Information and the operation of user interface must be understandable

Well written
Predictable formats
Input assistance

Robust - Content must be robust enough that it can be interpreted reliably by a wide variety of user agents, including assistive technologies

Maximize compatibility with current and future user agents