Web browsers fall under the category user agent, which the World Wide Web Consortium (W3C) has defined as follows:
Any software that retrieves and renders web content for users. This may include web browsers, media players, plug-ins, and other programs—including assistive technologies—that help in retrieving and rendering web content.
In order for web content to be accessible, web content designers must design accessible content, and user agents must be able to deliver that content in an accessible way. The W3C, through its Web Accessibility Initiative (WAI™), has developed guidelines for both parties. Guidelines for designers include the Web Content Accessibility Guidelines  and XML Accessibility Guidelines . Guidelines for user agents are the User Agent Accessibility Guidelines  (UAAG). A user agent that conforms to these guidelines will promote accessibility through its own user interface and through other internal facilities, including its ability to communicate with other technologies (especially assistive technologies). Furthermore, all users, not just users with disabilities, are expected to find conforming user agents to be more usable. Below are the twelve guidelines contained within UAAG 1.0:
- Support input and output device-independence
- Ensure user access to all content
- Allow configuration not to render some content that may reduce accessibility
- Ensure user control of rendering
- Ensure user control of user interface behavior
- Implement interoperable application programming interfaces
- Observe operating environment conventions
- Implement specifications that benefit accessibility
- Provide navigation mechanisms
- Orient the user
- Allow configuration and customization
- Provide accessible user agent documentation and help
The following AccessIT Knowledge Base articles may also be of interest:
-  Web Content Accessibility Guidelines
-  XML Accessibility Guidelines
-  User Agent Accessibility Guidelines
-  What is Web Accessibility Initiative (WAI)?
-  Which web browsers are most accessible?