Design
Using XSL and CSS to Format XML Documents

The birth of XSLT breaks down like this: Extensible Stylesheet Language (XSL) was conceived as an XML application for expressing stylesheets capable of manipulating XML documents. After its submission to the W3C in 1997, XSL split into two stylesheet standards: Extensible Stylesheet Language for Transformations (XSLT) and Extensible Stylesheet Language Formatting Objects (XSL-FO). XSLT is a language for transforming XML documents into typically other XML documents, HTML, and WML, but can also be used to produce documents in plain text and XSL-FO. XSL-FO is used to describe the layout of XML documents for printing, with the end product usually being in Portable Document Format (PDF). XSL-FO, a complex standard that some refer to as simply XSL, may eventually also be employed to lay out web pages, though this use is unsupported by current versions of web browsers. You can find more information about XSLT and XSL-FO, including their specifications, on the World Wide Web Consortium's web site, www.w3c.org. This column focuses on XSLT; later tutorials will expand on XSLT and delve into XSL-FO.
The early XSL proposals also gave rise to another breakaway standard, XPath. It grew out of the location-finding aspects of the XSL and XPointer specifications, which had been independently using similar mechanisms to find information in XML documents. XSLT heavily uses XPath to locate specific nodes or node sets within XML documents. Details about XPath specification can be found at http://www.w3c.org/TR/xpath.
XSL has another related standard, though unlike XSL it is not based in XML: Cascading Style Sheets (CSS). In the context of web publishing, Cascading Style Sheets (CSS) can also be used, at least theoretically, to statically format XML documents for display, but it cannot be used to transform them in any meaningful way. For example, XMetal, a tool for writing and editing in XML, uses CSS to format XML documents for display in its normal view. But until there is much stronger web browser support for displaying XML documents with CSS, it is not a practical option for web publishing.
CSS is not a competitor of XSLT. CSS is a method for statically formatting an XML document in a way that allows you to separate formatting and styling information from the document, but it does not allow you to dynamically transform an XML document into another data format. With CSS, you cannot manipulate the structure of an XML document, change the ordering of content, or dynamically generate a table of contents from a set of headings. Only XSLT can do that.
Using CSS to complement XSLT, however, is a powerful strategy for building web pages -- a strategy that splits presentation into what I call formatting and styling. Formatting can be seen to include basic HTML markup like headings, horizontal rules, lists and the like. Styling, meantime, defines the visual properties of markup: Its colors, sizes, widths, margins, bullet types, and so forth. Although the distinction between the two is not always clear-cut, formatting typically appears in the form of elements, styling information in the form of attribute-value pairs. For example, in <h1 style="color: olive;"> the h1 element formats the text as a first-level heading while the values of the style attribute, used for specifying inline CSS styles, reflect how the text should be styled.
You can gain a great deal of utility from separating as much styling information as possible from the formatting and placing it a Cascading Style Sheet, though until the second major version of CSS, CSS Level 2, is better supported by browsers, your HTML formatting markup will still have to retain some styling information, such as that for tables. Separating visual styling from formatting gives you a way to make wholesale design changes to a web site without having to change the formatting code in every HTML document; if you've set up your web pages properly, with all of them linking to a single CSS, you merely make the changes in one file, the Cascading Style Sheet. Cascading Style Sheets: The Definitive Guide, published by O'Reilly, provides a detailed account of how to use CSS. The W3C CSS specifications are available at http://www.w3c.org/Style/CSS/. In this tutorial, I will begin demonstrating how to use CSS with XSLT to separate styling from formatting, and I'll expand on the topic in later tutorials.