by Thomas Erl 		 XML Data Strategy
divider
 Replacing HTML
 Documents with XML
If you’ve been following the rise of XML, you will probably recall one of the earliest questions that arose when XML was just being introduced: “Will XML replace HTML?” In most cases, the answer consisted of an explanation about how XML is designed to supplement Web pages by adding meta information to the data being formatted by HTML (in other words, the answer was “no”).
While XML has been successfully used this way to date, in light of some recent developments this response will not remain valid for long. Supplemental XML technologies are being developed and positioned to replace core HTML functions, and it’s the original creators of HTML that are making it happen.
On the surface, XHTML looks a lot like HTML – pretty much the same syntax, the same tags. A goal of XHTML was not to provide more features, but to streamline the HTML syntax as much as possible in order to maximize efficiency and reduce the footprint required by (X)HTML processors (if you compare the XHTML 1.0 and 1.1 specifications, you will notice a trend towards reducing the quantity of HTML tags).
The primary objective of XHTML, however, is to provide the HTML syntax as an implementation of XML. By doing this, every XHTML document is automatically a valid, well-formed XML document. The XHTML language is simply an XML vocabulary, extensible like any other.
It is important to understand that XHTML is not a replacement technology for HTML, just a new (completely redesigned) version (call it HTML 5.0 if you like). It does, however, provide an important new framework that opens up the ability to incorporate a whole new generation of XML-based technologies.
(For the remainder of this paper, the terms XHTML and HTML are interchangeable. The focus is on how features of the base set of HTML tags, whether supplied in HTML 4.x or XHTML 1.x, are being replaced with new XML-related technologies.)
The prolific working groups at the W3C have been busy creating a series of specifications that supply an array of supplementary technologies specifically designed to enhance the overall feature-set of XML. Many of these enhancements give developers the ability to completely replace traditional HTML functionality with more powerful XML-based features.
Let’s look at some of these innovations, by doing a comparison of some HTML features and their new XML counterparts.
XHTML/HTML XML
Allows for the separation and isolation of formatting parameters using Cascading Style Sheets (CSS). Provides the same ability through the use of Cascading Style Sheets (CSS) or XSL style sheets.
Provides a syntax for navigating through a Web site’s physical or virtual directory structure. Provides the same ability through the use of XPath.
Provides the ability to assign hyperlinks to Web page text. Provides the same ability through the use of XLink.
Provides a set of tags for online data collection using forms. Provides the same ability through the use of XForms.
Some of the technologies listed in the XML column, such as XPath, XLink and XForms do much more than just provide the functionality already delivered by HTML – they consist of extensive specifications that supply large sets of features related to their core purpose. (Providing standard hyper linking capabilities, for instance, makes up only a fraction of XLink’s feature-set.)
As illustrated below, it is anticipated that HTML (and now XHTML) will gradually become a less prominent Web authoring language.
 

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Bit by bit, XML-driven innovations are supplanting core HTML functions with powerful extensions to the point that, in the foreseeable future, the Hypertext Markup Language could cease to exist entirely. In its place, we get an impressive Web authoring platform, rich with features that will allow us to create documents with accessible data, intelligent functionality, and fully integratable with the ever-growing world of XML.

   
   
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