As of 26-February-2015 HTML5 Image Description Extension (longdesc) is a W3C Recommendation.
Text alternatives are used to make visual information accessible by describing the information or function represented in an image. The alt attribute is designed to provide a concise description or functional equivalent for an image. For some images and contexts, more detail is required to convey all the information contained in the image. The longdesc attribute is designed as a means to provide this level of detail.
HTML 4 provided a longdesc attribute for the img element that enabled a machine-discoverable description of an image to be linked to the image. This extension specification defines a longdesc attribute to provide that functionality for HTML5.
I have always liked longdesc.
But then iamlost.