In HTML (Hypertext Markup Language), the "longdesc" attribute was specifically designed to allow for long descriptions of complex images. This contrasts with the "alt" attribute, which describes relatively simple images using a minimal amount of text. The "longdesc" attribute allows authors to link to a separate page (for example, longdesc="img1description.html") in which the image is described in detail. This is particularly useful for complex images such as graphs and charts.
The "longdesc" attribute was for a long time unsupported by assistive technologies, so workaround techniques (such as linking to the separate long description page using just the letter D as link text) were used in its place. However, recent versions of most screen reader applications now support the "longdesc" attribute. Screen readers typically announce the presence of a long description when available, and provide users with the option of reading it by executing a specified keystroke.
Since "longdesc" is now supported by screen readers, it is the recommended technique for providing long descriptions of images. However, in using "longdesc" be sensitive to the fact that there might still be some users with technologies that do not support it, and consider summarizing the most critical concepts from the image within the text content of the document. This practice helps all readers since it reinforces the author's ideas through multiple modes of presentation.