HTML5 Image Description Extension (longdesc)

W3C Last Call Working Draft

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Charles (McCathie) Nevile, ЯндексYandex


This specification defines a longdesc attribute (based on the longdesc attribute of HTML 4) to link descriptions to images in HTML5 content.

Note that by allowing a hyperlink inside another one, this document explicitly redefines the HTML concept of hyperlink in a limited set of circumstances.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document was developed through the HTML Accessibility Task Force, and is published by the HTML Working Group with approval by the Protocols and Formats Working Group as a Last Call Working Draft. If you wish to make comments regarding this document, please send them to public-html-a11y@w3.org (subscribe, archives). All feedback is welcome. Bugs can also be filed directly into the W3C Bug tracker for this specification. In order to be considered under the W3C Last Call Process, bugs must be filed or email sent to the mailing list by 16 September 2013.

Following feedback on the First Public Working Draft, a number of bugs were raised and resolved and a Second Public Working Draft was published for review. Review of this draft did not produce any new substantive issues, and a Call for Consensus of the Task Force to request this be published as the Last Call Working Draft passed without dissent.

Significant changes between successive public drafts are tracked in the Change History section.

This document is intended to become a W3C Recommendation, either on its own or merged into an HTML Recommendation.

Publication as a Last Call Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents


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.

<!-- The description is somewhere on the same page as the image -->
<img src="http://example.com/graph1" alt="(Drinks are getting sweeter)"
    title="Figure One" longdesc="#graph1Explained">

<!-- The description is a page on its own -->
<img src="figure1" alt="figure 1" longdesc="http://example.com/desc1">

<!-- The description is one of several within an external page -->
<img src="ExampleImage" alt="example" longdesc="http://example.com/descs#item3">

<!-- The description is included in a data: URI -->
<img src="logo" alt="W3C" longdesc="data:text/html;charset=utf-8;,%3C!DOCTYPE%20html%3E

1. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words must, should, and may in this specification are to be interpreted as described in [RFC2119].

1.1 Dependencies and definitions

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]

The terms "valid non-empty URL potentially surrounded by spaces", "hyperlink", and "reflect", used in this specification are defined in [HTML5], although this specification modifies the semantics of the term "hyperlink" in allowing hyperlinks to be nested in certain specific circumstances.

This document does not define the term "accessible" nor accessibility, but uses them with the sense they have in [WCAG]

2. Use Cases and Requirements

There are many ways users can successfully interact with images even if they cannot see, or see well. The alt attribute is designed to ensure that for everyday work, a user has enough information to replace an image, and often this is more helpful than a detailed description of every image. The longdesc attribute is designed to complement this functionality, to meet the following use cases.

Use Cases

Identifying a well-known image
Identifying an image is often important context for a discussion. There are many well-known images which are widely reproduced and well known. In some cases, such as "Mona Lisa", "Gerníka", Cubbin's "Lost", Leunig's "ramming the shears", the most effective way to identify an image is to use its title, creator, and similar information that would be found in a catalog. But different people know images by different titles, and some titles are not well known. Where the image itself cannot be seen, a description can often be used to offer easier recognition than the cataloging information, for example "Washington crossing some river, standing heroically in the boat, while soldiers do the hard work", or "the Da Vinci picture of the guy in a square and a circle".
Requires: Discoverability
Helped by: Inline, Reuse
Describing a complex diagram
In many environments diagrams are used to explain a concept or transmit information efficiently. For a user with a reduced ability to see the image (poor contrast vision and other conditions are as relevant here as a complete lack of vision), a description can enable that user to understand the information being presented.
Requires: Structured Markup, Inline, Reuse, Simple Return
Teaching Accessible Development
In many environments producing content that is accessible to users regardless of disability is a legal and/or market requirement.
Requires: Maintenance, Backwards compatibility
A self-describing artistic work
The visual design of many pages is an important part of the message the author intends to convey to a fully-sighted user. Many authors would like to convey as much of that design as possible to a user with low or no vision.
Requires: No visual encumbrance, Structured Markup
Helped by: Reuse, Simple Return
Referring to an existing description
Many well-known images are already described by other sources. The copyright on those sources may not be compatible with re-publishing the description, but there is little value in making a new one.
Requires: Reuse, Structured Markup, Simple Return, Optional Consumption
Helped by: No Visual Encumbrance
Linking to a description included within a page
If an image already has a description included within a page, making the linkage explicit can provide further clarity for a user who is not able to interpret the default layout. For example this happens when users force a re-layout of the page elements because they have magnified the content, or because they do not see the default visual relationship between the element and its description.
This practice also enables description to be provided for all users.
By keeping the association clear the content maintainer can more easily check that the description and link are actually correct.
Requires: Inline, Simple Return, Structured Markup, Optional Consumption
Localizing descriptions
When content is localized to multiple languages, it is important that metadata intended for human consumption, such as image descriptions, can be readily localized.
Helped by: Structured Markup, Reuse, Maintenance.

Requirements for an Image Description functionality

No visual encumbrance
It must be possible to provide a description for an image without forcing the description to be rendered on the page.
Structured Markup
It must be possible to include rich markup (e.g. HTML5) in the description of the image.
It must be possible to associate a description in the body of a page with an image in that page.
It must be possible to re-use a single description for multiple occurrences of an image.
It must be simple to maintain a library of images and descriptions for dynamic assembly, or dis-assembly and re-assembly, of content.
It must be simple for a user agent to automatically discover a description provided for a given image.
A user should be able to determine that there is a description available for a given image.
Optional consumption
A user must be able to choose not to read the long description of a given image.
Simple return
A user must be able to return from the description to the image.
Backwards Compatibility
It should be possible to use existing tools and techniques to associate an image with its description.

3. The longdesc attribute

Note that this extension changes the definition of a hyperlink in HTML, by allowing a longdesc attribute to occur inside a hyperlink.

3.0 Implementation

3.0.1 Authors and Conformance checkers

Zero or one longdesc attributes may be added to any img element.

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.

This section is not normative

Best practices for checking descriptions of images are beyond the scope of this document, but there are many resources available. For  further guidance tool developers can consult e.g. [ATAGB3]. ISSUE: This doesn't seem a great reference - is there anything better? This is tracked in bug 21501 and its dependent bugs. Unless a proposal for improvement is made and accepted this bug will simply be closed after Last Call.

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.

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

3.0.2 Authors

Authors should put descriptions within an element which is the target of a fragment link (e.g. longdesc="example.html#description") if a description is only part of the target document.

This section is not normative

Best practices for full descriptions of images are beyond the scope of this document, but there are many resources available.

3.0.3 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.

User agents should enable users to discover when images in a page contain a long description.

This section is not normative

longdesc can be present on an image within a hyperlink. This does not change the requirements on user agents, which must still enable the hyperlink to be followed.

Complete documentation of best practices for implementation is beyond the scope of this document. These notes are offered to help minimize the impact of common mistakes.

A common mistake in the past has been to use a description, instead of a URL that links to a description. Converting such values to data: URLs is a repair strategy for user agents that can help users to recover in cases where authors have made this mistake.

It is usually helpful when the behavior for finding, reading, and returning from a long description to the image described is a consistent experience.

Further information on making user agents accessible can be found in [UAAG].

3.1 Extension to the HTMLImageElement interface

partial interface HTMLImageElement {
             attribute DOMString longdesc;

3.1.1 Attribute

longdesc of type DOMString

The longdesc IDL attribute must be a valid non-empty URL potentially surrounded by spaces. It defines a hyperlink to a detailed description of the image its parent HTMLImageElement represents.

The longdesc IDL attribute must reflect the HTML content attribute longdesc.

This section is not normative

image.longdesc [ = value ]

Returns a DOMString that represents the attribute's contents.

Can be set, to replace the contents of the attribute with the given string.

/*Make the first internal longdesc reference absolute*/

var baseURL = document.location.origin + document.location.pathname
var someImage = document.querySelector('img[longdesc^=#]');

someImage.longdesc = baseURL + someImage.longdesc;
/*Open new windows for each longdesc found*/

var describedImages = document.querySelectorAll('img[longdesc]');

for (i in describedImages) {
  if (i.longdesc)
/*Tries to repair errors where the longdesc isn't a URI*/

var describedImages = document.querySelectorAll('img[longdesc]');

for (i in describedImages) {
  if (i.longdesc && !(validURL(i.longdesc)) { //assumes some URL validating function
    var theData = encodeURIComponent(i.longdesc);
    i.longdesc = "data:text/plain;charset=";
    i.longdesc += document.charset;
    i.longdesc += theData;

A. Acknowledgements

This section is not normative

Thanks to the HTML Working Group of the late 1990s for the original specification of longdesc, to those who have implemented it in various kinds of software, and to many many people involved with the development of HTML5 (including but not limited to those who discussed "ISSUE-30" in the HTML Working Group, the Protocols and Formats Working Group, the W3C Advisory Board, and around countless dinner tables, coffee breaks, and elsewhere) for the ideas, discussions and contributions that led to the initial draft of this specification. With the exception of Laura Carlson, who did far more very valuable work than it took me to produce this specification, I haven't named them: the list might be larger than the content of the specification.

For specific comments and suggestions that led to improvements over successive drafts of this specification, thanks to the W3C's HTML Accessibility Task Force, the W3C Internationalization Working Group, and to Jonathan Avila, Robin Berjon, James Craig, Paul Cotton, Steve Faulkner, John Foliot, Geoff Freed, Peter Gruzca, Richard Ishida, Anne van Kesteren, David MacDonald, Michelle McManus, Chris Mills, Jay Munro, Devarshi Pant, Marta Pawlowska, Silvia Pfeiffer, Wendy Seltzer, Leif Halvard Silli, Mathew Turvey, Klaas 'Z4us' V, and Boris Zbarsky. The code examples are rendered with Lea Verou's prism tools. Any errors are despite, not as a result of, their efforts.

B. References

B.1 Normative references

Robin Berjon et. al., eds. HTML5. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/html5/
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
Cameron McCormack. Web IDL. 27 September 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-WebIDL-20110927/

B.2 Informative references

Jutta Treviranus et. al., eds. Implementing ATAG 2.0, Principle B3 "Authors must be supported in improving the accessibility of existing content" October 2011. W3C Working Draft. URL: http://www.w3.org/TR/ATAG20-TECHS/#principle_b3
Ben Caldwell et. al., eds. Web Content Accessibility Guidelines 2.0, December 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-WCAG20-20081211/
Ian Jacobs et. al., eds. User Agent Accessibility Guidelines 1.0, December 2002. W3C Recommendation. URL: http://www.w3.org/TR/2002/REC-UAAG10-20021217/. A draft for a version 2.0 is available at http://www.w3.org/TR/UAAG20/

C. Change History

(This section is not normative)

Between this draft and the Second Public Working Draft

Editorial changes include:

  • Explicitly stating that longdesc makes a limited change to the semantics of the term "hyperlink" in HTML
  • Editing of the non-normative guidance in various places.
  • Explicitly adding a reference to the software license to allow people to re-use the code samples and IDL more easily with fewer restrictions.

Between the Second Public Working Draft and the First Public Working Draft

Substantive changes include:

  • Added a requirement that the destination of a longdesc should be "accessible".
  • The requirement to present content through the standard User Interface was changed from should to must.
  • The requirement to present content through the standard User Interface was narrowed to apply only to valid values of longdesc.
  • The requirement to present content through the Accessibility APIs was changed from should to must.
  • The requirement to present content through the Accessibility APIs was narrowed to apply only to valid values of longdesc.
  • The IDL attribute longdesc was changed to reflect the HTML content attribute longdesc.

Editorial changes include:

  • Clarifying that the document does not define "accessible", except that it is used in the sense that it is used in [WCAG].
  • Sectioning requirements according to whom they apply.
  • Removed the statement that User Agents may present descriptions which are erroneously included in the content instead of a link.
  • Added more examples