Provide text equivalents of auditory and visual content.
For some, pictures, sound bites, diagrams, videos and other media type data are inaccessible. Web developers should include text equivalents of all these forms of content so that those with disabilities or those without the proper browser can still access that content.
By providing text equivalents, you are making content available to those who depend on user agents such as screen readers and braille displays.
Implementing this guideline is fairly simple. "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," can all be given text equivalents by using the alt or longdesc function.
Videos without sound, or have sound but no narration, should be accompanied by audio tracks that explain the video. Another option for videos is creating closed captioning for the deaf. This will allow them see and read the video without having to read text seperate from the video.
By doing these text equivalents you make content available to just about everyone and provide a much more user-friendly experience.