Web Content Accessibility Guidelines
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¨Guideline 1
¨Provide equivalent alternatives to auditory and visual content.
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–Provide content that, when presented to the user, conveys essentially the same function or purpose as auditory or visual content.
Although some people cannot use images, movies or sounds directly they may still use the pages that include equivalent information to the visual or auditory content. For example text equivalent information for an an image of an upward pointing arrow that links to the table of contents  could render the text as go to table of contents. In some cases the text equivalent should describe the appearance of the graphic for exaplle the description of ascii art or circuit diagrams. For those with hearing difficulties, subtitles or transcription  on video clips serve as equivalent.