XHTML: The Transition From HTML to XML

By Samuel Gallard, Junior Consultant (sgallard@techmetrix.net)

Following in the footsteps of HTML 4.0, Extensible Hypertext Markup Language(XHTML) 1.0 became an official W3C standard on January 26, 2000. XHTML 1.0 is the re-formulation of HTML 4.0 as an Extended Markup Language (XML) application, but it remains compatible with all browsers that support HTML 4.0. XHTML is designed to perform in two areas in which HTML proves quite limited: language extensibility and portability.

What's the Purpose of This New Standard?

HTML has been enabling the exchange of hypertext data via the Internet since the W3C made it a standard in 1990. However, HTML comes up short when it comes to today's new wireless technologies.

HTML is Not Extensible
New technologies, in particular Web-related technologies, are in constant evolution. The lasting power of these technologies thus depends on their ability to integrate new functionality quickly and easily. The Web has also developed considerably and has now entered the generation of so-called intelligent Web sites (sites that are capable of establishing a user profile and personalizing site content in relation to this profile).

Conversely, HTML, which is the standard for document presentation, has evolved relatively little. Why is this? Firstly, it is possible that such evolution was quite simply unnecessary. Secondly, HTML's structure prevents it from evolving with ease. Indeed, HTML is made up of a fixed set of tags and defines the standards that browsers interpret in order to represent HTML data. In other words, HTML imposes a given presentation, and adding a tag that authorizes a new presentation feature requires a new version of the language.

HTML is Not Portable to Wireless
The W3C predicts that by 2002, 75% of all Internet access will be from devices other than PCs (mobile phones, personal digital assistants, TV, watches and so on). Yet, the current version of HTML cannot be ported to small devices, partly because the document presentation possibilities are obviously not the same on a 17-inch screen as on that of a mobile phone and partly because the bandwidth of a WAP-enabled telephone is far from being sufficient enough to display data such as images and other multimedia files (sound, video). As a result, some tags are not optimized for hand-held devices and others are entirely unwarranted at this time. TOP

What Does XHTML Have That HTML Doesn't?

XHTML Benefits From XML's Strengths
Version 1.0 of XHTML is defined as HTML 4.0's re-formulation as an XML application. As such, XHTML inherits XML's major assets, which are its extensibility and its portability.

XML Concepts

1. Like HTML, XML stems from the Standard Generalized Markup Language (SGML). While HTML represents a set group of elements, XML is a meta language without a fixed collection of tags that makes it possible to write customized and open description languages, which we will call XML applications. Thus, XML is extensible.

2. One of XML's particularities is that it separates data from its presentation. XML describes document structure independently from its presentation (a style sheet defines data display). XML can therefore be ported to any compatible application or device.

The following elements are needed to write an XML application: an XML sheet containing the data (text, images, sound, and so on); an XSL or Cascading Style Sheets (CSS) style sheet defining how data must be presented; and possibly, a Document Type Definition (DTD), which defines the grammar of the elements that can be used in an XML sheet. A DTD is not mandatory because any valid XML document is inevitably well-formed. We will look at the significance of this term when we present the syntactic rules below. Furthermore, if a DTD is mentioned, XML must validate it.

3. Actually, XML makes up a group of interoperable standards among which we find the following:

• Extensible Style Language (XSL): This language is used to transform XML and HTML data into other presentation formats. In this way, it is possible to separate data presentation. XSL is a subgroup of CSS. CSS are used to represent simple XML because they can only present information in the order in which it was received.
  
  
• Extensible Linking Language (XLL): This XML linking language provides links in the same way as HTML, but with more power. For example, XLL links can be multi-directional and can designate an object rather than just a page.
  
  
• Also, Mathematical Markup Language (MathML) for describing and using mathematical expressions; Resource Description Framework (RDF) for metadata; Synchronized Multimedia Integration Language (SMIL) for multimedia data;and Scalable Vector Graphics (SVG) for 2-D vector graphics.

XHTML Integrates XML Concepts

XHTML has the same structure as an XML application. It includes an XHTML sheet including the data and the elements related to its presentation, which is handled by HTML tags. There may be an XSL or CSS style sheet; however, as XHTML remains HTML-compatible, style can still be defined within the document using the <style> tag. Finally, XHTML's structure includes one of the three DTDs corresponding to the three types of HTML documents.

As part of the XML family, XHTML can support the other XML standards defined above. TOP

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