W3C

HTML Image Description Extension (longdesc)

Exit Criteria for Candidate Recommendation Research and Reference

Authoring Requirements

The longdesc attribute must contain a valid non-empty URL potentially surrounded by spaces. The URL is a hyperlink to a description of the image that the parent img element represents.

The linked description should be in a broadly accessible format.

When a description is only part of the target document, authors should link to a fragment link in the target document (e.g. longdesc="example.html#description").

Authors should not rely solely on longdesc where standards exist to provide direct, structured access.

Examples of well-formed, accessible longdesc that also point to fragment links

In a review and verification process of Laura Carlson's longdesc research, I identified 9 different sites with 48 pages referencing 89 longdesc fragments links (republished below). A quick assessment of the rest of her research revealed at least 13 additional sites, with 48 pages and 73 fragment links.

2012 Canada's Emissions Trends Report: 5 pages, 19 Fragment Examples

The pages above are also available in French and could be considered for inclusion

ESC XV - Club Sport Loisir - Paris 15: 1 page, 1 fragment link
Indiana University-Purdue University Indianapolis Common Core Curriculum Committee: 1 page, 2 examples

It's fair to assume, based on the format of the longdesc page, there are more pages on this site that can be used as examples.

Lambda Theta Phi Latin Fraternity: 8 pages sharing the same fragment

All of the following separate pages share the same long description page: Long Description: Lambda Banner

It's fair to assume, based on the format of the longdesc page, there are more pages on this site that can be used as examples.

Texas State Library: 16 pages, 16 fragment links
U.S. Department of Health and Human Services: 1 page, 6 fragment links
Capita Annual Report: 1 page, 1 fragment
Centers for Disease Control and Prevention (CDC): 3 pages, 31 fragments

The Centers for Disease Control also uses longdesc on images in a smoking and tobacco use analysis for all fifty US States. The following are examples for just two of the fifty states - New York and California:

Environment Canada: 12 pages, 12 fragments

July 2011 Canada's Emissions Trends Report:

Authoring Tools

This section is informative

Best practices for checking descriptions of images are beyond the scope of this document, but there are many resources available. For further guidance authoring tool developers can consult e.g. Appendix A-3: Long text descriptions of [ATAG].

High quality implementation in authoring and content management tools is likely to include checking for common errors, such as including a description instead of a URL in the attribute, or pointing to a resource that is an image or plain text, which are often poor choices for a high-quality description. Additional examples of error checking and repair options can be found in Appendix B: Levels of Checking Automation and Appendix C: Levels of Repair Automation of [ATAG]

This document does not define the term "accessible", nor accessibility. For further guidance on making an accessible document, authors can consult e.g. [WCAG].

John Foliot has researched support for Longdesc in contemporary Authoring Tools. Research was performed in May 2011.

User Agents

If the longdesc value is valid, User agents must make the link available to all users through the regular user interface(s). 

If the longdesc value is valid, User agents must expose the link to relevant APIs, especially accessibility-oriented APIs.