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Technologies in the presentation layer; adding intelligence to transformations in a multitier environment.

Article Excerpt
There are many different ways to implement XML-based technologies in applications. Understanding the history and background of technologies will aid in becoming an architect of them. Before there was HTML there was a metalanguage called SGML (Standard Generalized Markup Language). As a metalanguage, it was designed to be a language for describing other markup languages. One such markup language created from SGML was HTML. I personally have never programmed in SGML. From what I've heard and read it's a very involved and costly resource to use, but built to last. Either way, it seems that SGML wasn't a language that ordinary developers could easily pick up and use.

XML, written as a lighter version of SGML, was designed as a metalanguage that could be used on a wider scale. It's based on the profile of SGML, but not to the same complexity. Where HTML uses SGML, XML is just SGML on a smaller scale. From XML we've seen languages such as XSL (which can be divided into XSLT and XSL-FO), SVG (Scalable Vector Graphics), WML (Wireless Markup Language), and XHTML (Extensible Hypertext Markup Language). I commonly refer to these as the X-based languages. There have been implementations of these X-based languages in the Java and COM world that include considerable compliance with the W3C recommendation with custom extensions.

HTML became widely used very quickly. During its popularity, a lot of nonprogrammers began to create Web pages. A good portion of these nonprogrammers probably had little idea where HTML came from and how to properly write valid HTML. A lot of browsers have been programmed to be very forgiving of malformed HTML. I've heard speculation that an estimated 80% of the code base in Internet Explorer or Netscape Navigator is to render malformed HTML.

With the ever-growing popularity of XML over the last couple of years, many parsers have evolved in the marketplace for the Java and COM environments. One of the first and most popular for the Web has been XSLT, a transformation language that enables transformation of an input XML document into another XML-based output structure such as XHTML. This is a remarkable development for the Web and provides substantial benefits over HTML. HTML includes presentation and data in one document. With XSLT, programmers can use an XML datastore that comes from a database or is created as a standalone document and apply a stylesheet to define how to display the data.

This separation of presentation and data documents allows one XML data source document the ability to have different XSLT documents that can be applied to that one XML document. One XSLT document may transform the data source into XHTML. Another may transform the same data source into WML. Reusable components improve the efficiency of any application.

One issue with XSLT is that what's contained in it must conform to XML (because XSLT is still an XML document). Now consider HTML. The more recent versions aren't XML conformant. So how do we create an XSLT document and in it contain HTML? The answer is simple: XHTML. XHTML was written from XML and is therefore XML compliant. XHTML ensures only that the HTML follows good markup techniques. For example, in HTML "
" elements don't have to be closed. In XHTML we'd show the same element as "

" or as an empty element "
". Most developers using X-based technologies are more likely to use XHTML, even where not explicitly necessary.

Design Considerations

Microsoft has done a wonderful job of keeping bleeding-edge technology for XSLT in the hands of developers while XSL was in working draft status. Hence, the namespace of earlier versions of the Microsoft parser using XSL contained the letters WD, for working draft, as follows:



The W3C eventually decided to split XSL into two areas -- XSLT for transformations and XSL-FO for formatting objects. Microsoft's new namespace for XSLT supports virtually all of the recommendation from the W3C and no longer includes the letters WD, as shown below:



Using the correct namespace in XSLT is crucial to the XSL stylesheet's performing properly. It's probably the No. 1 response...

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