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Technical Review

Why XML?
By Simon St. Laurent
XML Editor

A new technology promises to standardize, stabilize, and improve the Web. Whether or not you know, (or even need to know), anything about the Extensible Markup Language, or XML, it is going to have a profound effect on the way that Internet communications are conducted.

XML is essentially a set of rules for creating documents and a set of rules for creating descriptions of documents. The rules for creating documents will be reasonably familiar to HTML users and very familiar to SGML, (Standard Generalized Markup Language), users. XML is much stricter about syntax in many ways: tags must match in case, all attributes must use single or double quotes, and all elements must nest cleanly - no overlapping elements are permitted. (For example, bold italic just italic is forbidden - use bold italic just italic instead.

Markup is still created with tags like the familiar


though XML takes enforcement of tag placement much more seriously . Every start-tag must have an end-tag, or, if the element created by the tags is empty (like IMG and BR in HTML), must use a special syntax that ends with /> instead of >. HTML's
; becomes .

Although XML may seem like a heartless grinch to HTML coders used to applying endtags only when they felt like it, leaving quotes off elements, and generating code that worked well enough, there are enormous benefits to be reaped from this discipline. The browser vendors will no longer have to mimic each other's bugs, and will need to do much less processing than they did with HTML. Authoring tools can work from simpler tree-like structures instead of data streams, and should have a much better change of understanding code created using a different authoring system.

XML is much more than a syntax straitjacket for HTML, however. It provides a set of rules for creating new markup standards. The W3C is moving to an XML-based model for HTML using XML to define modules for HTML. The Synchronized Multimedia Integration Language (SMIL, pronounced 'smile'), which allows developers to create much more sophisticated multimedia presentations for delivery over the web uses XML as its foundation, as does the new MathML standard for presenting mathematical equations. The World Wide Web Consortium is moving wholeheartedly to XML as the basic architecture for future development.

Netscape and Microsoft are moving to support XML as well. Both companies plan to include XML in the 5.0 releases of their browsers, and partial support is available in the developer editions currently available. XML also brings with it a set of supporting standards for styling, linking, and managing XML documents. XML will also be able to take advantage of Cascading Style Sheets in the next browser releases, allowing developers to create (or apply) new markup languages that reflect their content while still making everything look presentable in a browser or on paper.

Intrigued? For more, take a look at my XML: A Primer or any of the XML books starting to appear from publishers. I've also written several essays, available at

Simon St. Laurent is a web developer, network administrator, computer book author, and XML troublemaker working in Ithaca, NY.  His books include XML: A Primer, Dynamic HTML: A Primer, and Cookies.

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