A good starting point is the Web Content Accessibility Guidelines from the World Wide Web Consortium's Web Accessibility Initiative.
Boston's WGBH is considered an outstanding model of accessible web design. It's Rich Media Center at http://ncam.wgbh.org/richmedia/ offers information about making streaming, animation and intereactive web content accessible. The U.S. government requirements that it's information technology be accessible. This is known as Section 508 of the 1998 Rehabilitation Act.
Computer companies have developed accessibility guidelines, for example Sun's Java Guidelines http://java.sun.com/products/jfc/accessibility/ and Java Accessibility Utilities. (See also http://www.sun.com/tech/access/access.quick.ref.html) and an IBM site offering guidelines for accessible applets: http://www-306.ibm.com/able/accessjava.html.
Apple was the first company in the industry to support disability access. Their products are described at http://www.apple.com/accessibility/. Apple has integrated a magnifier and spoken interface in Os X. Microsoft's site is http://www.microsoft.com/enable/. Adobe provides free accessibility tools for reading .pdf files. Macromedia flash now allows userrs of their authoring tools to create accessibe flash. Adobe's best practices whitepapers for pdf and flash authors make it clear that the toolmakers cannot create an accessible web without the participation of designers and developers. Although The American Foundation for the Blind has been working with Adobe to improve the accessibility of pdf documents. However many creaters of pdfs scan in an image of text (rather than text itself) which cannot be read with screenreaders. Optical Character Readers can read pages and images of text allowed, but often cannot handle the crooked and blurred pages that result from hurried scans of documents into pdf.
Until recently, accessibility in linux involved a great deal of work for users. A HOWTO was available, a mailing list and archives were maintained by the BLINUX Project, and a full-featured (though complicated) text interface was available through the Emacspeak audio desktop for Linux. More recently, an accessibility project is being supported by Sun for the Gnome desktop as part of StarOffice software for linux. The ubuntu linux distribution includes accessibility features. Their website has an extensive list of linux accessibility resources.
A 2005 list of web browsers and screen readers designed for persons with disabilities shows the diversity of clients available for the disabled. The World Wide Web Consortium is currently working on Accessibility Guidelines for the next generation of browsers and readers.
Physically impaired computer users use a variety of hardware interfaces to meet their needs. An example is Origin Instruments' single switch input device. Videos of people using single switch controls are distributed via YouTube by Assistiveware.Designing for accessibility takes time, which makes it expensive unless toolmakers integrate accessibility into the main functions of their tools instead of making it an option. The trend to widespread commercial use of handheld devices to access the web has created a demand for pages that look good on a small screen. Often these look good to screenreaders too. Lorelle VanFossen's design guidelines for photographers shows the effects of different screen sizes (including PDAs), fonts, and colors. Web designers often mistakenly assume that designing for people with disabilities means giving up multimedia and the "latest" design tools (and fads). Multimedia pages can be accessible, as these examples show:
To see if your pages are standard HTML, you can validate them with the Worldwide Web Consortium's Tidy tool, which can be downloaded from Sourceforge for most operating systems. The online version of the Bobby tool can be used to validate one page per minute for either the World Wide Web Consortium's Web Content Accessibility Guidelines 1.0 or the U. S. government's Section 508 compliance.