Web Accessibility Guidelines are Split into 3 Priorities

Priority 1 – covers basic web accessibility requirements.

* Provide a text equivalent for every non-text element (e.g., via “alt”, “longdesc”, or in element content). This includes: images, graphical representations of text (including symbols), image map regions, animations (e.g., animated GIFs), applets and programmatic objects, ascii art, frames, scripts, images used as list bullets, spacers, graphical buttons, sounds (played with or without user interaction), stand-alone audio files, audio tracks of video, and video.

* Ensure that all information conveyed with color is also available without color, for example from context or markup.

* Clearly identify changes in the natural language of a document’s text and any text equivalents (e.g., captions).

* Organize documents so they may be read without style sheets. For example, when an HTML document is rendered without associated style sheets, it must still be possible to read the document.

* Ensure that equivalents for dynamic content are updated when the dynamic content changes.

*Until user agents allow users to control flickering, avoid causing the screen to flicker.

* Use the clearest and simplest language appropriate for a site’s content.

Priority 2 – removes significant barriers to accessing Web documents.

* Ensure that foreground and background color combinations provide sufficient contrast when viewed by someone having color deficits or when viewed on a black and white screen.

* When an appropriate markup language exists, use markup rather than images to convey information.

* Create documents that validate to published formal grammars.

* Use style sheets to control layout and presentation.

* Use relative rather than absolute units in markup language attribute values and style sheet property values.

* Use header elements to convey document structure and use them according to specification.

* Mark up lists and list items properly.

* Mark up quotations. Do not use quotation markup for formatting effects such as indentation.

* Ensure that dynamic content is accessible or provide an alternative presentation or page.

* Until user agents allow users to control blinking, avoid causing content to blink (i.e., change presentation at a regular rate, such as turning on and off).

* Until user agents provide the ability to stop the refresh, do not create periodically auto-refreshing pages.

* Until user agents provide the ability to stop auto-redirect, do not use markup to redirect pages automatically. Instead, configure the server to perform redirects.

* Until user agents allow users to turn off spawned windows, do not cause pop-ups or other windows to appear and do not change the current window without informing the user.

* Use W3C technologies when they are available and appropriate for a task and use the latest versions when supported.

* Avoid deprecated features of W3C technologies.

* Divide large blocks of information into more manageable groups where natural and appropriate.

* Clearly identify the target of each link.

* Provide metadata to add semantic information to pages and sites.

* Provide information about the general layout of a site (e.g., a site map or table of contents).

* Use navigation mechanisms in a consistent manner.

Priority 3 – provides even greater access to Web documents.

* Specify the expansion of each abbreviation or acronym in a document where it first occurs.

* Identify the primary natural language of a document.

* Create a logical tab order through links, form controls, and objects.

* Provide keyboard shortcuts to important links (including those in client-side image maps), form controls, and groups of form controls.

* Until user agents (including assistive technologies) render adjacent links distinctly, include non-link, printable characters (surrounded by spaces) between adjacent links.

* Provide information so that users may receive documents according to their preferences (e.g., language, content type, etc.)

* Provide navigation bars to highlight and give access to the navigation mechanism.

* Group related links, identify the group (for user agents), and, until user agents do so, provide a way to bypass the group.

* If search functions are provided, enable different types of searches for different skill levels and preferences.

* Place distinguishing information at the beginning of headings, paragraphs, lists, etc.

* Provide information about document collections (i.e., documents comprising multiple pages.).

* Supplement text with graphic or auditory presentations where they will facilitate comprehension of the page.

* Create a style of presentation that is consistent across pages.

Web Accessibility Tool
http://www.contentquality.com/

This is a great online tool which generates a web accessibility report for your web site based on:

Section 508 guidelines (http://www.access-board.gov/508.htm)
Priority 1, 2, 3 guidelines

Complete List of Web Accessibility Evaluation Tools
http://www.w3.org/WAI/ER/tools/complete

Conclusion

Most site owners will not implement these guidelines because it involves a lot more work which means investing more money.
Some web designers may include some of the guidelines while designing a site and point out to the site owner the extra expenses involved if they want their site to be completely web accessible.

If you want to build your site for long term success and reach a wider audience I’d suggest making it web accessible.