Use the longdesc attribute for images with complex meaning
When images are complex or provide a lot of information, use of alt text alone may not be sufficient to convey their meaning. For example, a graph showing temperature trends in Los Angeles over the last century could have an alt attribute of "Graph of Los Angeles Weather 1900-1999." The problem is that the user learns there is a graph but does not understand the content, which visually expresses temperature change across time.
- Use the LONGDESC attribute to provide descriptive information for images whose meaning cannot be conveyed with only a title. The LONGDESC attribute links to a file that contains only the descriptive information relating to the image. It can be any length necessary to accurately convey meaning.