XML Accessibility Guidelines

W3C Working Draft NOTE 19 June 2001

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Daniel Dardailler, W3C (dd@w3.org)
Sean B. Palmer (sean@mysterylights.com)


This document explains how to design accessible applications using XML, the Extensible Markup Language. Compared to the HTML or MathML languages, XML is one level up: it is a meta syntax used to describe these languages, as well as new ones. As a meta syntax, XML provides no intrinsic guarantee of device independence or textual alternate support. It is essential, therefore, that XML formats and tools designers are provided with guidelines that explain how to include basic accessibility features - such as those present in HTML, SMIL, and SVG - in all their new developments. Thus, the XML GL builds upon the model established by the Web Content Accessibility Guidelines (WCAG), the Authoring Tool Accessibility Guidelines (ATAG), and the User Agent Accessibility Guidelines (UAAG) as a means of providing such guidance.

Status of this document

This document is a WAI PF Internal Working Draft Note made available by the Protocols and Formats Working Group (PFWG) for review by the W3C membership.

Publication of this paper does not imply endorsement by the W3C membership. A list of current W3C technical reports and publications, including working drafts and notes, can be found at http://www.w3.org/TR/.

Please send general comments about the content of this document to the PF mailing list: w3c-wai-pf@w3.org. Send minor editorial comments directly to the editors.

Note that the priorities as assigned to the checkpoints are provisional, and under consideration by the group.

Table Of Contents


XML (eXtensible Markup Language) is a meta-syntax, used to create new languages.

It can be seen as a simplification of SGML (Standard Generalized Markup Language), designed to promote a wider acceptance in Web markets, but serving the same functionality of extensibility and new language design.

HTML (HyperText Markup Language), on the other hand, is one particular application of SGML, which covers one set of needs ("simple" hypertext documents) and one set of element and attributes.

For instance, in HTML, authors can write elements like:-

 <TITLE>XML and Accessibility</TITLE>
 <ADDRESS lang=fr>Daniel Dardailler</ADDRESS>

and they can only use elements (TITLE, H1, etc) defined by the HTML specification (which defines about a hundred), and their attributes.

In SGML and XML, authors can define their own set of elements, and end up with documents like:-

 <MENU>New England Restaurant</MENU>
 <APPETIZER>Clam Chowder
   <PHOTO url="clam.jpg">A large creamy bowl of clam showder, with
       bread crumbs on top</PHOTO>

which may fit more closely the needs of their information system.

Within W3C, the HTML language is now being recast as XML - this is called XHTML - including a modularization of HTML to suit the needs of a larger community (mobile users, Web TV, etc).

XML is therefore not to be seen as a replacement of HTML, but as a new building layer on top of which HTML is to be placed, next to other languages designed by W3C, such as MathML (for representing mathematical formula), SMIL (for synchronizing multimedia), SVG (for scalable graphics), etc., and other new languages designed by other organizations (such a OpenEBook, XML-EDI, etc.).

Furthermore, it is important to understand that XML is not only a User Interface technology (like HTML), but can and is often used in protocol communication, to serialize and encode data to be sent from one machine to another.

XML Grammars, and The Scope Of XMLGL

The XML grammars (called schemata - but see the caveat about our use of the term "schema" in the definition section) can be classified along two different axes:-

Tagsets for: User Interface (UI)--oriented structural textual rendering, such as Docbook, HTML, MenuML, OEB, etc.; specialized rendering - for example MathML, Scalable Vector Graphics (SVG), MusicML, Synchronized Multimedia Integration Language (SMIL); or any generic data storage format. An informal definition is 'anything for which the question "is there a textual equivalent of all rich media data bits?" makes sense'. Data-centric schemata include both the interaction and behavioural aspects of an XML application.
When the content being marked up is metadata. Examples: For expressing data processing (for example XSL - Extensible Style Language), RDF (Resource Description Framework), Schema languages, etc.

According to this taxonomy, these guidelines only address Data-oriented schemata. This does not imply that there are not accessibility issues or features in a Metadata-Oriented schemata - see, for example, how XSLT, a component of XSL, can assist in Braille formatting. Since they do not convey data, however, Metadata-Oriented schemata are out of the scope of these guidelines.

Problem statement

The WAI (Web Accessibility Initiative) has done extensive work in the HTML area, resulting in lots of new functionalities being added to the version 4.0 of the language (see the HTML4 Accessibility Improvements paper).

These features includes:

One area of concern with the advent of XML is that the freedom of design it brings has and can result in a loss of accessibility features, present today because of HTML's pervasive presence and widely available specification.

For instance, one could design a new XML language that would prevent the creation of accessible documents, by not including in the element or attribute set a way to attach an alternate textual description for a photo:-

 <MENU>New England Restaurant</MENU>
 <APPETIZER>Clam Chowder
   <PHOTO url="clam.jpg"/> <!-- no alt attribute or 
                                   textual content model here -->

In this example, the problem is not that the author of this document didn't put an alt attribute value attached to the PHOTO element, it's that the designer of the language didn't put the attribute in the language itself (that is, in the schema).

But let's start by defining what we mean by accessible schema and documents (Details on these definitions are provided at the end of this document):-

As explained in the introduction, we're only considering Data-oriented languages here, and for them, the message is simple: be device independent and export your semantics as much as you can.

While the priority is stronger on the first aspect (multi-modality), both aspects are important, as without the knowledge of the meaning of the XML elements and attributes, there is little chance that alternative user agents can do something intelligent with just the document bits.

This semantics knowledge can be provided through human readable documentation of course, but having machine readable assertions of semantics that can then be used to present the document in various media is paramount for pervasive access (that is, you don't need a programmer, you just need a program). Enabling others to map from your language to exisiting ones, or vice versa, is a useful accessibility feature.

ICADD (International Committee on Accessible Document Design) was a pioneer in this topic, for SGML accessibility and ways to convey arbitrary schema semantics (using specific SGML binding mechanisms). A few years later, ICADD has not really been adopted (in fact, the ICADD DTD was replaced by HTML and its well known semantics), and people are still trying to solve the same problem, albeit with more experience in the field of HTML accessibility, and applied to XML this time.

Guidelines for Designers of Data-oriented XML Tagsets

This section provides a list of four abstract guidelines. Some examples of checkpoints are provided, and detailed checkpoints and techniques that schema designers can follow to achieve accessibility when designing new XML schemata still have to be defined by WAI and W3C.

An additional piece of advice we give to schema designers is that in their specification itself (the documentation) they always emphasize the accessibility features of their new language and try to include accessibility as part of any conformance statement that they introduce (be it for the document themselves, or for readers/editors of the language). See the Conformance section of the SVG specification, and Writing Accessible SVG for an example of both practices.

Caveat About Presentational & Formatting Tagsets

Languages used only for presentation to a certain scope of users (that is, final form tagsets) should adhere to the following caveats:-



In the presentation of guidelines for XML accessibility, we try to separate abstract guidelines from implementation techniques. This allows us to talk about the general guideline principles without spending the time up-front to solve the implementation issues.

In fact, there are several techniques for achieving the same result and people's decision will be a function of time and product available and their own commitment to access.

For instance, if an XML designer want to create some kind of "list" element in a given markup, this can be implemented using various techniques:

Along with the choice of the metadata mechanism and vocabulary comes the issue of semantics availability: how does one access the schema and possible XSLT or schemata from an instance document? This is sometimes referred to as XML packaging or related-resource discovery and is a very important feature for accessibility.

XML accessibility definitions:


Geoff Freed, Al Gilman, Vijay Gummadi, Ian Jacobs, Chris Lilley, William Loughborough, Charles McCathieNevile, Stephane Maes, Eve Maler, Dave Pawson, Gregory J. Rosmaita, Henry S. Thompson, Carlos A. Velasco, Norman Walsh.


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