Longdesc attribute

In HTML, longdesc is an attribute used within the image element, frame element, or iframe element. It is used to reference a long description website of the image, frame, or iframe in question.

While longdesc was designed to be used by screen readers to display image information for computer users with accessibility issues, such as the blind or visually impaired, it is actually seldom used for this purpose, because there are relatively few authors who use the attribute, and most of those authors use it incorrectly.

Example
Content of :

Linking to the long description in the text
Since very few browsers and screen readers do make the link available at all, it is advisable to include a link to the description page near the  element.