Websites built with accessibility in mind are flexible in meeting different user needs, preferences and suituations. Though these methods can increase usability for everyone who uses the web they are often legally required to be implemented in a specific effort to prevent discrimination against people with disabilities.
These are some readable introductions to accessibility that cover; what accessibility is, why it is important, as well as practical advice.
- Web Accessibility Initiative's Introduction to Web Accessibility
- Mark Pilgrim's Dive into Accessibility
- Joe Clark's Building Accessible Websites book
Standards, guidelines and legislation
- SENDA - Special Educational Needs and Disability Act/Bill
- Disability Discrimination Act 1995
- Section 508
- Web Accessibility Initiative
- Web developer extension for Firefox
- Accessibility extension for Firefox
- Validators (for HTML,CSS and RSS)
- Cynthia Says accessibility checker
see also this long list of accessibility tools.
- Web Standards.org's Accessibility Task Force Manifesto
- Accessibility articles from A List Apart
- Mark Pilgrim's Won’t somebody please think of the gerbils?
- Joe Clark's writings on accessibility
- Wikipedia article on Web Accessibility
- Validity and Accessibility