HTML is the most popular markup language in the world. The goal though of most HTML markup is appearance—the HTML tags do not provide you with any information. For example, if you see an HTML document with an element that has content as simple as <td>12345</td>, you do not know what the content represents. The content could be the zip code of a particular town, or it might be the population of the town.
An XML document:
Contains information that is marked up with tags that describe all the pieces of information
Models the relationships between all the pieces of information
Is contained in a single element called the root element which becomes the root of a tree structure that contains other elements that represent the information
An XML document might include the element <zipcode>12345</zipcode>, and you know from the zipcode tag that 12345 is a zip code.
XML documents separate the content from the presentation, and they can be transformed (using XSLT, the Extensible Stylesheet Language for Transformations) into a variety of presentation types such as:
An HTML page that includes <td>12345</td>
A PDF file that includes zip code information
A display of zip code information in wireless phones or pagers
With XSLT, you can transform XML documents into other documents, which are often XML documents themselves. For example, Web pages created in XHTML (an XML-compliant version of HTML) are XML documents, and you can use XSLT to transform any XML document into a styled XHTML Web page for display in a browser.
A cascading style sheet (CSS) allows you to add style rules to the elements of a document that define how the content of the elements should be rendered. Using a CSS enables you to separate the contents of an HTML, XHTML, or XML document from its visual presentation. However, XSLT moves you beyond CSS because XSLT offers you complete flexibility to change the layout of content. XSLT also allows you to define rules that not only alter the design, but also add, change, or remove elements of the content if appropriate.
For an overview of XML, see the first section of the chapter on exporting and importing XML in the PowerBuilder Users Guide. For detailed information about XML and XSLT, see the O’Reilly and Associates, Inc. Learning XML and XSLT books.
