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For an image that presents important information, such as a chart, table, or diagram, it may be necessary to provide a detailed description of the image's content. This description supplements the alternative text and can be included in the text of the main web page, or it can be placed in a separate page for this purpose and referenced by the "longdesc" attribute of the IMG element. Index. For example:
<IMG SRC="chart.gif" ALT="Chart of cash flow for each month" LONGDESC="http://www.thismachine.com/cashflowchar.txt">
Most current web browsers do not yet support the "longdesc" attribute. In order to provide for both the present and the future, we recommend using both the "longdesc" attribute and following the image by a descriptive (or D) link. The D link, a convention established by the CPB/WGBH National Center for Accessible Media, allows you to link manually to the target of the LONGDESC by linking a small "D" positioned near the image. For example:
<IMG SRC="chart.gif" ALT="Chart of cash flow for each month" LONGDESC="http://www.thismachine.com/cashflowchar.txt"> <A HREF="cashflowchar.txt">D</A>
Computers cannot interpret images and present them in a meaningful alternate format; a long description gives the computer something to present to the user. This is important for users who have turned off image-loading in their web browsers, those using text-based browsers like Lynx, and people who are visually impaired and require the use of a screen reader to read the contents of the screen for them.
Section 508, Part 1194.22, Paragraph a
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