DIAGRAM Center

Diagram Center Proposal

Introduction

External descriptions in HTML5 and EPUB 3

EPUB 3 currently lacks a mechanism to support external descriptions of graphical and other material ("infographics"). While the work on specifying technologies and conventions for intrinsic accessibility of infographics progresses, the need for a well-defined external description mechanism remains:

• Image descriptions which are needed by people with vision disabilities cannot typically be contained within the length limitations of alt attribute.
• Rich image descriptions that involve tabular data or lists cannot be properly represented within the alt attribute
• A mechanism for alternative representations of images, such as tactile images, is needed.
• Additional use cases can be found at: http://www.d.umn.edu/~lcarlson/research/ld.html#uc

At the time of writing, the HTML5 specification on which EPUB 3 is based does not provide a satisfactory solution to this need:

• The longdesc attribute has been made obsolete, and while it may be reinstated in future drafts of HTML5, this attribute (in its original HTML 4 definition) suffers from a restricted usage context; in HTML5, external descriptions are applicable to a much wider range of elements than img and frame.
• The aria-describedBy attribute is limited to identifying targets that are fragments within the the same document as the object being described.

As such, this document constitues a solution proposal to Issue 193 in the EPUB Working Group Issue Tracker.

Relationship to WAI-ARIA

The attribute described in this document is intended to serve as an interim solution until a native integration of an equivalent attribute occurs in a future version of WAI-ARIA. At the time of writing, the development of a definition for this attribute (prototypically referred to as aria-describedAt) is still in early inception stages within the W3C.

The epub:describedAt attribute

All elements in EPUB 3 XHTML Content Documents may have the epub:describedAt attribute set. The epub:describedAt attribute's value is used by the Reading System to locate a resource that contains extended descriptions of the element on which epub:describedAt occurs.

If set, the epub:describedAt attribute's value must be a single IRI.

Reading Systems may employ user interface hints to alert the user that an extended description is available for the given element. Depending on the application context, Reading Systems may attempt to automatically retrieve the referenced resource, or may attempt retrieval only after User confirmation.  It is recommended that a described element also have an ID attribute that enables the Reading System to easily return to the described element.

This document does not impose limitations on the media type(s) of the resources referenced from epub:describedAt. While it is anticipated that the DIAGRAM Center Content Model will be used in some deployment contents, there are other contexts where other media types may be preferable.

The method with which the information in the referenced resource(s) is presented to the User is beyond the scope of this document.

Examples

References

[HTML5]

HTML5: A vocabulary and associated APIs for HTML and XHTML

[RFC3987]

Internationalized Resource Identifiers (IRIs), M. Dürst, M. Suignard. IETF.

Discussion

Please discuss or leave feedback and questions at: http://diagramcenter.org/templates/forum/viewforum.php?f=10