XSL The Extensible Style Language
Styling XML Documents
By Norman Walsh
From the earliest days of the Web, we've been using essentially the same set of tags in our documents. Web pages written in HTML use HTML tags and the meaning of those tags is well understood: <H1> makes a heading, <IMG> loads a graphic, <OL> starts an ordered list, and so on. The number of tags has slowly grown, and there have been numerous browser-compatibility issues, but the basic tag set is still the same.
There's a significant benefit to a fixed tag set with fixed semantics: portability. A Web page that uses the standard tags can be viewed by just about any browser, anywhere in the world. However, HTML is very confining; Web designers want more control over presentation and many processes would benefit from more descriptive tagging.
Enter XML. With XML, we can use any tags we want. We can write documents using our own tag names -- names that are meaningful in the context of our subject matter and offer the possibility of far greater control over presentation. But this freedom comes at a price: XML tag names have no predefined semantics. An <H1> might just as legitimately identify a tall hedge as a first-level heading. Is <IMG> an image, or an imaginary number? Who knows?
The style sheet knows. From the very beginning of the XML effort, it was recognized that in order to successfully send XML documents over the Web, it would be necessary to have a standard mechanism for describing how they were to be presented.
