By Trisha Gorman
XML stands for Extensible Markup Language. Spearheaded by the World Wide Web Consortium (W3C), XML became a formal specification in mid-February 1998.
XML developers will tell you that XML isn't a language but rather a system for defining other languages. You may have already heard of, or even used, one of these other languages --Microsoft's Channel Definition Format (CDF) for push, for example.
The W3C, which is working on a slew of XML-related recommendations, calls XML "a common
syntax for expressing structure in data." Structured data refers to data that is tagged
for its content, meaning, or use. For example, whereas the <H1> tag in HTML specifies
text to be presented in a certain typeface and weight, an XML tag would explicitly identify
the kind of information: <BYLINE> tags might identify the author of a document,
<PRICE> tags could contain an item's cost in an inventory list--all the way down to
<DOGFOODBRAND> if that's the level of detail required.
By separating structure and content from presentation, the same XML source document can be written once, then displayed in a variety of ways: on a computer monitor, within a cellular-phone display, translated into voice on a device for the blind, and so forth. It will work on any communications devices that might be developed; an XML document can thus outlive the particular authoring and display technologies available when it was written.
So XML will have a life outside of the Internet, serving the publishing industry at large, for example, and especially people who produce documents intended to appear across multiple media. Some large-scale document publishers who have been using Standard Generalized Markup Language (SGML) for years will convert to XML. Still, platform-independent XML was developed for the Web, and that's where it will have the most impact.
The DOM
XML's real strength for the Web is how it interacts with the Document Object Model (DOM), an interface that defines the mechanisms for accessing data in a document.
Using the DOM, programmers can script dynamic content in a standardized way. In other words, they can use it to cause a specific piece of content in a browser's document tree to behave in a certain way, creating a small effect--for example, a piece of text might turn blue when a user mouses over it. Both Netscape Navigator and Microsoft Internet Explorer have their own proprietary DOMs, but both companies say they will support the W3C standard DOM in the next versions of their browsers.