“The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect.”
– Tim Berners-Lee, W3C Director and inventor of the World Wide Web
The Web Content Accessibility Guidelines (WCAG) were developed through the W3C process in cooperation with individuals and organizations around the world, with a goal of providing a single shared standard for web content accessibility that meets the needs of individuals, organizations, and governments internationally.
WCAG are stable, referenceable technical standards. Each standard has 12-13 guidelines that are organized under 4 principles: perceivable, operable, understandable, and robust. For each guideline, there are testable success criteria, which are at three levels: A, AA, and AAA. 
Any image included in your online content should include Alternative Text or a Null Tag.
Any audio, video, or audio-video content included must include Captions (and possibly a Transcript).
Users must be able to separate the foreground from the background of your content, this is accomplished through careful usage of color. Also, because users may need to increase the size and contrast of the page, the text should not be presented as an image.
Use proper Content Markup so that users can navigate your site (or document).