XML in a Nutshell, 3rd Edition

Transformation and Presentation

The markup in a typical XML document describes the document’s structure, but it tends not to describe the document’s presentation. That is, it says how the document is organized but not how it looks. Although XML documents are text, and a person could read them in native form if they really wanted to, much more commonly an XML document is rendered into some other format before being presented to a human audience. One of the key ideas of markup languages in general and XML in particular is that the input format need not be the same as the output format. To put it another way, what you see is not what you get, nor is it what you want to get. The input markup language is designed for the convenience of the writer. The output language is designed for the convenience of the reader.

Of course this requires a means of transforming the input format into the output format. Most XML documents undergo some kind of transformation before being presented to the reader. The transformation may be to a different XML vocabulary like XHTML or XSL-FO, or it may be to a non-XML format like PostScript or RTF.

XML’s semiofficial transformation language is Extensible Stylesheet Language Transformations (XSLT). An XSLT document contains a list of template rules. Each template rule has a pattern noting which elements and other nodes it matches. An XSLT processor reads the input document. When it sees something in the input document that matches a template rule in the stylesheet, ...