XML Standards News;
Web Accessibility, XHTML & Canonicalization

Web Content Accessibility Guidelines Become a PR

The Web Content Accessibility Guidelines, explaining how to make Web content accessible became a Proposed Recommendation on March 24, 1999. The review period for this recommendation will end on April 21. The guidelines are intended to guide page authors and Web site designers, as well as developers of authoring tools in making content accessible. Following these guidelines, content should be more available to all users and all user agents (such as voice browser, mobile phone, or desktop browser). While certainly useful to those with disabilities, guidelines also provide benefits to those of us that operate under constraints such as noisy environments, highly lighted rooms, or when we must work with our hands and cannot use a mouse. Guidelines do not limit the use of graphics, images, or video. Rather they explain how to make this content available under all conditions.

It is important to note that the guidelines in the document do not address any specific browser technology, as that technology tends to change rapidly. Rather it provides general guidelines. Specific recommendations related to specific browser technologies can be found on the Web Accessibility Initiative site. The Proposed Recommendation is part of the Web Accessibility Initiative.

More Work on XHTML

Last month we reported that XHTML 1.0, a reformulation of HTML 4.0 as an XML 1.0 application, and three namespaces corresponding to the ones defined by HTML 4.0 was posted in development by W3C on February 24, 1999. XHTML differs from HTML in two significant ways. First, although XHTML is based on the current HTML tag set, it is designed to be extensible. This extensibility relies upon the XML requirement that documents be well-formed. Under SGML (hence HTML), the addition of a new group of elements would mean alteration of the entire DTD. In an XML-based DTD, all that is required is that the new set of elements be internally consistent and well-formed to be added to an existing DTD. So XML greatly eases the development and integration of new collections of elements into a tag set such as HTML. Second, XHTML is designed for portability. As of March 4, XHTML moved into final review as a working draft.

In addition, a new XHTML posting appeared on March 6. This Working Draft specifies a modularization of XHTML 1.0 into semantic modules and implementation of the modules through a DTD. The semantic modules provide a means for subsetting and extending XHTML. The DTD modularization improves the ability to create complete new DTDs from XHTML and other DTD modules. The partitioning of the XHTML model into semantic modules was implemented by two primary methods.

The first method is the use of parameter entities. This specification classifies parameter entities into six categories and names them consistently using:

• .mod (DTD module that is a separate file entity)
• .module (mechanism to control use of a DTD module by containing either of the conditional section keywords INCLUDE or IGNORE)
• .content (represents the content model of an element type)
• .class (represents elements of the same class)
• .mix (represents a collection of element types from different classes)
• .attrib (represent attribute specifications within an ATTLIST declaration)

The second method used to create semantic modules is the creation of standard DTD fragments. These fragments are used to encompass the markup declarations of a specific semantic component or "feature", from higher-level document features like tables and forms, to lower-level components such as block elements, inline elements, and display elements. Modules can contain modules, creating a hierarchical structure mirroring the document model.

The notion of "plug and play" with DTD modules is very attractive, and is at the heart of XHTML. Complex document models often resort to extensive parameterization of semantic modules to facilitate understanding, markup reuse, extensibility, and maintenance. The resultant modules may have many interdependencies, and may require a fair amount of "rewiring" when adding or removing a DTD module. In light of this, a compromise must be made between markup flexibility, complexity of the DTD, and ease of maintainability.

The XHTML DTD attempts to control problems that result from interdependencies caused by this parameterization of semantic modules by localizing many of the more "global" parameter entities to several "common" modules that are declared early in the DTD.

You can learn more about the modularization of XHTML by linking to http://www.w3.org/TR/xhtml-modularization.

XML Canonicalization Requirements

On March 7, a note was posted that outlined the design principles, scope and requirements for the Canonicalization of XML being developed by the World Wide Web Consortium's XML Syntax Working Group. Canonicalization is based on the idea that it is possible for logically equivalent XML documents to differ in their physical representation. For example, two logically equivalent documents may vary in the order of attributes within a tag. These documents, while logically equivalent are not physically equivalent. Because a physical difference may exist, equivalence testing cannot be done at the byte level. Since equivalence testing is critical in the arenas of digital signatures, checksums, version control and conformance testing, a way to test equivalence between XML documents at the syntactic level and, in particular, by byte-for-byte comparison is the aim of the Canonical XML specification. This specification will describe how a canonical form of XML documents can be constructed such that logically equivalent documents will have the same byte-for-byte representation.

You can learn more about canonicalization by linking to http://www.w3.org/TR/NOTE-xml-canonical-req.

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