Introduction to XSLT

In this chapter and the next, we're going to introduce the use of XSLT  Extensible Stylesheet Language Transformations  to format and display XML documents. In this chapter we'll give you an overview of XSLT, and how to use it in the client's browser with some real-world examples of transformations. Chapter 6 will then develop this knowledge with some more complex examples of XSLT in action. You will then learn about using XSLT to transform XML on the server in Chapters 8 to 11.

An extreme example of XSLT can be found at http://people.w3.org/maxf/ChessGML/ (you'll need the SVG plug-in from http://www.adobe.com/). Here, an XML representation of a chess game (stored as ChessGML, a standard XML vocabulary for representing chess games in XML) is converted into an SVG animation using XSLT. SVG is an XML-based standard for describing images and animations, and the XSLT stylesheet used by this site converts a simple ChessGML game into a fully-animated representation of the game being played move by move.

So what is XSLT? Essentially, it is a way of transforming XML documents into other documents (often other XML documents). When it comes down to it, XHTML web pages are just XML documents, so we can use XSLT to transform an XML document into a styled XHTML web page, and display it in a browser.

We have already seen how XML can be displayed using CSS. XSLT goes one stage further by giving you complete flexibility to change the layout of your content. Furthermore, it allows you to define rules that not only alter the design, but can also add, change, and remove elements of the content, if appropriate. So if you want to exclude some content from a page based on your XML, XSLT can automatically strip out this content for you. Conversely, XSLT can add in content or generate new content based on the existing XML document, such as headers and footers. You can go further still with XSLT, and convert your content into alternative formats such as RTF (Rich Text Format) documents and CSV (Comma Separated Value) files for Word and Excel.

Although XSLT is very powerful, CSS is often better for simple tasks. We're certainly not suggesting that, having just read the CSS chapter, you throw all of your CSS knowledge away and use XSLT instead. It is often best to use a combination of CSS and XSLT to achieve the look we want.

We will be focusing on the most practically useful aspects of XSLT, and by the end, you should not only have an understanding of how to use XSLT, but more importantly recognize why XSLT is useful and know where to use it.

Server-side versus Client-side

There are two approaches to applying XSLT to your XML content. You can perform these XSLT transformations server-side on your web server, and serve up the resulting HTML web page to a browser. Chapters 8-11 of this book will cover server-side XSLT transformation in detail.

The other way to use XSLT is to send a source XML document and an XSLT stylesheet to a browser, and ask the browser to apply the XSLT transformation for you. Here we will learn which browsers support this client-side application of XSLT, see what benefits it gives, and see some examples of client-side XSLT in action.

Browser Support for XSLT

Given the inconsistencies between HTML support in the different browsers, it should come as no surprise that they offer varying support for XSLT as well. The good news is that through careful use of XSLT it is possible to achieve support by more than 95% of the browsers on the market while still providing readable content for the rest.

There are only two different XSLT specifications to worry about. The recent browsers with XSLT support (Internet Explorer 6, Netscape 6+, and Mozilla) all support the XSLT 1.0 standard  the specification for this is available online at http://www.w3c.org/TR/xslt. Unfortunately Internet Explorer, in versions 5 and 5.5, as well as PocketEI, support an earlier form of the XSLT standard  the XSL Working Draft (see http://www.w3.org/TR/1998/WD-xsl-19981216, although only the transformations section of this specification is implemented). We'll see how to support both soon.

The picture is muddied further because the older IE browsers can be updated to use more recent XSLT processors. In practice only a tiny fraction of users, mostly developers, will have updated, and they will have almost certainly have upgraded their actual browsers by now.

Throughout the rest of the book, to make things less confusing we'll be referring to the XSL Working Draft implemented by IE 5.0 and IE 5.5 as XSL-WD, and the full version of XSLT 1.0 supported by IE 6, Netscape 6+, and Mozilla as XSLT 1.0. XSLT 1.0 is also the version of XSLT implemented by the server-side XSLT processors described later in this book.

For the most part, XSL-WD is a subset of the functionality available in XSLT. For a list of the exact differences between XSL-WD and XSLT, see the table at the end of the chapter, as well as http://www.zvon.org/xxl/XSLTreference/Output/index.html. Most browsers on mobile devices do not support XSLT, with the exception of the Pocket PC, which supports XSL-WD.

The examples in this chapter will work across all of IE 5+, Netscape 6+, and Mozilla, with only very slight changes for XSL-WD browsers. Chapter 6 will introduce some of the features of XSLT 1.0, and also some of the Microsoft-specific extensions.

As explained in Chapter 3, even if the client doesn't support the level of XSLT that you would like to use, you can still get many of the benefits of XSLT by using server-side script to do browser version sniffing, and then styling on the server for older clients and sending XML/XSLT to the browser for newer clients.  The same XSLT 1.0 stylesheet can be used server-side and client-side.