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