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Use Cases for Extended Descriptions

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This is a page to identify the various use cases for long descriptions in order to facilitate the development of an engineering solution that meets them all.

Some of the following are from the longdesc extension spec for HTML5. There are more in ISSUE-30 from the HTML WG.

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".

Alternative techniques for well-known images

The same effect can be achieved with many standard and supported features in HTML, SVG, and EPUB, including but not limited to:

  • standard links
  • inline frames
  • figure captions
  • footnotes in EPUB

Several of these techniques, particularly links and EPUB footnotes, are already discoverable and well-supported in mainstream browsers and ebook content readers.

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.

Alternative techniques for complex diagrams

The same effect can be achieved with many standard and supported features in HTML, SVG, and EPUB, including but not limited to:

  • image maps
  • MathML
  • SVG (augmented with WAI-ARIA to be accessible)

In the cases where it is appropriate to use MathML or accessible SVG, using those technologies is recommended.

Teaching Accessible Development

In many environments producing content that is accessible to users regardless of disability is a legal and/or market requirement.


This section may be irrelevant. No one is arguing things should not be made accessible. This section ("Teaching Accessible Development") may need to be removed as accessibility does not rely on any particular technique.

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.

Alternative techniques for self-describing artistic work

The same effect can be achieved with many standard and supported features in HTML, SVG, and EPUB, including but not limited to:

  • standard link to a site colophon
  • footnotes in EPUB

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.

Alternative techniques for using existing descriptions

The same effect can be achieved with many standard and supported features in HTML, SVG, and EPUB, including but not limited to:

  • standard links
  • inline frames

These techniques are already discoverable and well-supported in mainstream browsers.

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.

Alternative techniques for linking to a description included within a page

Note: one alternative answer is in the title of the section. If you need "linking", use a link.

  • standard link to an in-page fragment identifier (e.g. "#foo")
  • aria-describedby

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.


This section may be irrelevant. True, localization is important, but long descriptions provide no benefit for localization. You'd have to localize using other techniques, too.

DAISY Use Cases

Brief Introduction

Many elements in a publication need additional information to be accessible to persons with disabilities. Not only blind and low vision users, but persons with other disabilities greatly benefit from additional information. In fact, we envision that many people that do not have disabilities can benefit from additional information. The fundamental work on what can be in an access enhanced graphical content can be viewed at: http://www.diagramcenter.org

aria-describedby use cases

Library serving persons with disabilities, content republishers, place age, grade, and language specific descriptions transformed into HTML5 in an aside element. Aria-describedby provides theID to this supplemental materials. The user agent notifies the person that additional information is available, and the person can elect to read this additional material.

A publisher has additional information about an element they want to provide, but does not want this to interrupt the normal flow of the page. They place the information into HTML5 and store it as a file available in the package. They use the src in a hidden iframe to be loaded as the page loads. The user agent based on the aria-described attribute alerts the person that additional information is available.

Use cases for external accessible descriptions: aria-describedat use cases

Content republishers use an online repository for descriptions of recurring images/infographics (e.g. the periodic table or the map of Africa). The online repository allows for resource sharing among publishers and republishers. The online repository also enables the descriptions to be enhanced over time without requiring client-side content updates to give the user access to the enhancements.

A publisher wants to adapt the content that is used for accessible descriptions to the particular user, using axes such as age, language, and preferred alternate formats (e.g. tactile graphics). The publisher uses external descriptions to allow the most suitable content to be returned dynamically through negotiation, instead of having to ship all versions of the description to each individual user.

A textbook republisher needs to ship a book to a student asap and does not currently have the time to create accessible descriptions for all included images/infographics. The republisher therefore initially includes only stub links to an external repository, ships the book, and then adds the descriptions incrementally at the server side.

Annotation Use Cases

Note: Annotations are yet to be defined. It is expected that this work will start in 2013 following the completion of EPUB 3.0.1.

A library serving persons with disabilities identifies the images in a textbook that require more than what was provided by the publisher. A set of annotations are prepared and then distributed to integrate with the EPUB 3 document. The user agent integrates annotations associated with the particular locations and alerts the person that an annotation is available.