The use of Extensible Markup Language (XML) to transfer and process data on the Internet is increasing. Much of this increase is due to companies embracing Web services for their applications and the growing popularity of Ajax.

This book provides information technology professionals with a good foundation in XML concepts. The introduction to XML starts with an explanation of XML syntax. To be valid, an XML document must be well formed. A well-formed document must meet several criteria. For example, it must contain a single document element, all document elements must be closed correctly, and attribute values cannot be empty and must be surrounded by quotation marks.

XML is used to create document markup languages. It isn’t a language itself, but is used to construct other languages, for instance, the pervasive Extensible Hypertext Markup Language (XHTML). Languages written with XML are called vocabularies, and several are specifically designed for use on the Web. According to the World Wide Web Consortium (W3C), XHTML is HTML reformulated with XML. It is an improvement over HTML, requiring page markup to comply with XML syntax rules. Valid XHTML documents separate presentation from content, improving accessibility and processing time, and streamlining site maintenance. Web developers can validate XHTML documents using online tools like the W3C markup validation service (http://validator.w3.org).

XML is not used in isolation, but is used with other recommendations from the W3C. These related recommendations include XML namespaces, providing information to identify each vocabulary used in a document. The vocabularies are defined with document type definitions (DTDs) and XML schemas.

XML documents can be displayed using cascading style sheets (CSS) or extensible stylesheet language (XSL). There are limitations to using CSS. XSL is more flexible, allowing content to be reordered, sorted, or filtered. The two parts of XSL are XSL transformations (XSLT) and XSL formatting objects (XSL-FO). XSL-FO is mainly used for printed output. XSLT is used to transform and format content as an XHTML document, or even as a comma delimited file for use by a spreadsheet or database.

Mathematical Markup Language (MathML) was developed to display mathematical expressions on the Web. The W3C MathML group divided it into two types of encoding: presentation and content. Presentation MathML is used to control the display of mathematical notation by Web browsers. Content MathML is designed to be processed by a MathML engine.

Scalable vector graphics (SVG) is designed to describe two-dimensional graphics and graphical applications in XML. Web professionals can use SVG to create shapes, include images on a page, and represent text using some of the same techniques used with shapes (for example, color gradient).

Really simple syndication provides the ability to publish news feeds. This capability is used by companies like the Associated Press, as well as by smaller Web sites.

Several chapters are devoted to a more in-depth discussion of the concepts introduced up to this point. The chapter topics include: client-side XML, displaying XML using CSS, and client-side XSLT. These are followed by a discussion of writing scripts, specifically JavaScript, to use the W3C XML document object model (DOM) to process XML documents. Web professionals will have to overcome differences between how Internet Explorer (IE), which uses ActiveX objects, and the other major browsers interact with DOM, however. To complicate things even more, different versions of IE use different ActiveX objects.

The solution presented in the book is to use a wrapper to allow the browsers to exhibit the same JavaScript actions. The book’s resource file, available from the publisher’s Web site, provides the xDOM wrapper for use in the book’s examples.

There is a short introduction to Ajax concepts. Ajax uses technologies that have been around for years, but that have been recombined as a new approach to developing Web applications. Even though it has increased in popularity, Ajax has problems, like all Web technologies developed to this point. An example the author gives is that Ajax can break a browser’s “Back” button functionality, causing usability problems.

The author then discusses how to use Adobe (formerly Macromedia) Flash to display XML data. This is followed by an introduction to server-side XML. The book closes with two case studies on creating XML applications. The first uses .NET to create an XML news application. The second case study uses the open-source PHP language to create an XML community weather portal application. Both case studies demonstrate the basics of developing a simple XML application from beginning to end.

I recommend this book for anyone with little or no experience using XML. However, the reader will need to reference other resources. Throughout the book, the author provides uniform resource locators to resources found on the W3C Web site.