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Why not stick with HTML?
At some point in any XML conversation, the question about XML's perceived predecessor — namely, the HyperText Markup Language, or HTML — inevitably comes up. The reason why it doesn't make sense for many organizations to stick with HTML, even for documents already in that format, is that HTML suffers from two fundamental, insoluble problems by virtue of its design and capabilities:
Because HTML is essentially a "closed" SGML-based application, HTML markup cannot be extended without altering the standard DTD that describes this markup language. Although browser vendors haven't shied away from implementing proprietary markup (or markup extensions) anyway, that technique violates the formal description of HTML markup in a standard DTD.
HTML's markup elements — the language components that define containers or modifiers for document content — cannot capture the kinds of semantic information that XML applications can capture. In essence, HTML must remain general in purpose and scope, and cannot therefore capture the semantics of a family tree or a set of patient records at all well. Because XML can model such types of data directly and abstractly, it enables applications to use their knowledge of data structures, relationships, and constraints on specific data elements or attributes to help control data input, and to query and represent captured data in all kinds of interesting ways.
By permitting XML documents to carry the descriptions of themselves along with their contents, the standards group behind both HTML and XML is providing a big, tasty carrot to software vendors and users alike to persuade them to switch from HTML to XML — namely, the ability to parse, represent, and manipulate documents and data in much more powerful and meaningful ways than HTML can even provide. In fact, XML is so powerful that data captured in some XML notation can easily be accessed and manipulated in native form, then transformed into HTML for delivery to Web browsers that aren't so adept at handling XML as such. Until user agent software catches up with XML more completely, XML also provides a way to bridge the technology gap between XML and HTML with only minimal additional effort. Other transformations can grab the same data and turn it into PDF files or other document formats, plain text, or even files suitable for display on a PDA or cellular phone.
To learn more about all of these topics, and other interesting items of XML lore, please consult the "Top 5 XML Resources" list at the end of this article.