XML Reference Guide

HTML is terrific for what it's designed for: creating pages that adjust to the browser or other environment in which the content is to be displayed. But what if you wanted more control over the presentation of your content? Yes, you can use Cascading Style Sheets to perform all sorts of formatting miracles, but there comes a point at which you really need to move on to a technology more suited for the job.

In the XML realm, that technology is XSL-FO, or Extensible Stylesheet Language Formatting Objects. (Current versions refer to it simply as XSL, as opposed to XSLT, for Extensible Stylesheet Language Transformations, but XSL-FO seems to have stuck, no matter what the W3C says.) XSL-FO provides a methodology and a number of elements that enable you to design a document that not only controls the presentation of content, but also enables you easily convert your content to formats such as paged documents (perhaps the most common use of XSL-FO) in such a way that content not only flows smoothly from page to page, but also adapts to situations such as odd-and-even page differences.

For example, we can create a short, center-bound book of the "What is XML?" section of this guide:

<?xml version="1.0" encoding="iso-8859-1"?>

<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <fo:layout-master-set>
    <fo:simple-page-master master-name="my-page" page-height="4in">
      <fo:region-body margin=".5in" />
    </fo:simple-page-master>
  </fo:layout-master-set>

  <fo:page-sequence master-reference="my-page">
    <fo:flow flow-name="xsl-region-body">
      <fo:block>A cousin of Hypertext Markup Language (HTML),
          Extensible Markup Language (XML) provides a human-readable, 
          platform-independent way to represent data, whether
          it's content destined for a web page or brochure, or data 
          destined for an enterprise database.

          Like HTML and their mutual parent, Standard Generalized 
          Markup Language (SGML), XML works on the principle of tags, 
          which delineate elements and content. Consider, for example, 
          the following XML document:

          &lt;?xml version=&quot;1.0&quot;?&gt;
          &lt;candy&gt;
             &lt;product&gt;Chocolate Bar&lt;/product&gt;
          &lt;/candy&gt;

          This simple document has two elements: candy and
          product. These elements are indicated by tags, or 
          constructions that start with the less-than sign (&lt;) and 
...
    </fo:flow>
  </fo:page-sequence>
</fo:root>

This formatting objects file has two major sections. First, we define the page layout master, which includes the size of the page itself and margin information. Second, we have the actual page sequence, which specifies the master to follow. The data automatically flows between pages, as shown in Figure 1.

Of course, this is the equivalent of putting HTML on the page without breaking it up using <p></p> tags. We can break the content up into blocks, and we can specify style information on inline sections using the inline element:

<?xml version="1.0" encoding="iso-8859-1"?>

<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <fo:layout-master-set>
    <fo:simple-page-master master-name="my-page" page-height="4in">
      <fo:region-body margin=".5in" />
    </fo:simple-page-master>
  </fo:layout-master-set>

  <fo:page-sequence master-reference="my-page">
    <fo:flow flow-name="xsl-region-body">
      <fo:block padding-top="10px" padding-bottom="10px">A cousin 
           of Hypertext Markup Language (HTML),
           Extensible Markup Language (XML) provides a human-readable, 
           platform-independent way to represent data, whether
           it's content destined for a web page or brochure, or data 
           destined for an enterprise database.</fo:block>
      <fo:block padding-top="10px" padding-bottom="10px">Like HTML 
           and their mutual parent, Standard Generalized Markup Language
           (SGML), XML works on the principle of tags, which delineate 
           elements and content. Consider, for example, the following XML 
           document:</fo:block>
      <fo:block font-family="monospace">
           &lt;?xml version=&quot;1.0&quot;?&gt;</fo:block>
      <fo:block font-family="monospace">&lt;candy&gt;</fo:block>
      <fo:block font-family="monospace">
           &lt;product&gt;Chocolate Bar&lt;/product&gt;
      </fo:block>
      <fo:block font-family="monospace"></fo:block>
      <fo:block font-family="monospace">&lt;/candy&gt;</fo:block>
      <fo:block padding-top="10px" padding-bottom="10px">This simple 
           document has two elements: <fo:inline 
           font-family="monospace">candy</fo:inline> and
           <fo:inline font-family="monospace">product</fo:inline>. 
           These elements are indicated by tags, or constructions that 
           start with the less-than sign (&lt;) and end with the 
           greater-than sign (&gt;) surrounding the name of the element. 
           In other words, the <fo:inline 
           font-family="monospace">candy</fo:inline> element starts 
           with the <fo:inline font-family="monospace">candy</fo:inline> 
           opening tag:</fo:block>

       <fo:block font-family="monospace">&lt;candy&gt;</fo:block>
...
    </fo:flow>
  </fo:page-sequence>
</fo:root>

The content can be broken into block elements, just as it is in HTML, with style information added directly to the element as attributes, rather than as part of the style attribute. Similarly, the inline element enables you to add style information without having to create a new block, as shown in Figure 2.

You can also use XSL-FO to create "conditional" page masters. For example, say we wanted to bind this booklet. We might want to leave extra margin in the center by creating different page layouts for the odd and even pages:

<?xml version="1.0" encoding="iso-8859-1"?>

<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <fo:layout-master-set>
    <fo:simple-page-master master-name="odd-page" page-height="4in">
      <fo:region-body margin=".5in" margin-left="1.5in" />
    </fo:simple-page-master>

    <fo:simple-page-master master-name="even-page" page-height="4in">
      <fo:region-body margin=".5in" margin-right="1.5in" />
    </fo:simple-page-master>

    <fo:simple-page-master master-name="first-page" page-height="4in">
      <fo:region-body margin="2in" margin-right="1.5in" />
    </fo:simple-page-master>

    <fo:page-sequence-master master-name="booklet-sequence">
      <fo:repeatable-page-master-alternatives>
        <fo:conditional-page-master-reference page-position="first"
                                              master-reference="first-page"/>
        <fo:conditional-page-master-reference odd-or-even="odd"
                                              master-reference="odd-page"/>
        <fo:conditional-page-master-reference odd-or-even="even"
                                              master-reference="even-page"/>
      </fo:repeatable-page-master-alternatives>
    </fo:page-sequence-master>
  </fo:layout-master-set>

  <fo:page-sequence master-reference="booklet-sequence">
    <fo:flow flow-name="xsl-region-body">
      <fo:block padding-top="10px" padding-bottom="10px">
            A cousin of Hypertext Markup Language (HTML),
            Extensible Markup Language (XML) provides a human-readable, 
            platform-independent way to represent data, whether
            it's content destined for a web page or brochure, or data 
            destined for an enterprise database.</fo:block>
...
    </fo:flow>
  </fo:page-sequence>
</fo:root>

In this case, rather than following a single page master, the content flows into the conditional page master, which chooses the appropriate page master based on which page it is, as shown in Figure 3. You can also specify page positions such as "first" to include a title page.

You can also add static content to the page using a specific region. If the content flow takes up more than one page, this static content is automatically be added to each appropriate page:

<?xml version="1.0" encoding="iso-8859-1"?>

<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
  <fo:layout-master-set>
    <fo:simple-page-master master-name="odd-page" page-height="4in">
      <fo:region-body margin=".5in" margin-left="1.5in" />
      <fo:region-start background-color="#EEEEEE" extent="1.25in" />
    </fo:simple-page-master>

    <fo:simple-page-master master-name="even-page" page-height="4in">
      <fo:region-body margin=".5in" margin-right="1.5in" />
      <fo:region-end background-color="#EEEEEE" extent="1.25in" />
    </fo:simple-page-master>

    <fo:page-sequence-master master-name="booklet-sequence">
      <fo:repeatable-page-master-alternatives>
        <fo:conditional-page-master-reference odd-or-even="odd"
                                              master-reference="odd-page"/>
        <fo:conditional-page-master-reference odd-or-even="even"
                                              master-reference="even-page"/>
      </fo:repeatable-page-master-alternatives>
    </fo:page-sequence-master>
  </fo:layout-master-set>

  <fo:page-sequence master-reference="booklet-sequence">
    <fo:static-content flow-name="xsl-region-end">
        <fo:block padding=".25in" margin=".1in">NOTES</fo:block>
    </fo:static-content>
    <fo:static-content flow-name="xsl-region-start">
        <fo:block padding=".25in" margin=".1in">NOTES</fo:block>
    </fo:static-content>

    <fo:flow flow-name="xsl-region-body">
      <fo:block padding-top="10px" padding-bottom="10px">A cousin 
         of Hypertext Markup Language (HTML), Extensible Markup Language (XML)
         provides a human-readable, platform-independent way to represent 
         data, whether it's content destined for a web page or brochure, or 
         data destined for an enterprise database.</fo:block>
...
    </fo:flow>
  </fo:page-sequence>
</fo:root>

Static comment can be added to a region and will appear on each page that accompanies the flow, as shown in Figure 4.

Notice that I've added a region-start, which appears to the left of the body (in a left-to-right language) to the even pages, but not to the odd pages, and vice versa for the region-end. This way, I can specify static content for those particular regions, and they'll only appear on the pages for which that region has been specified.

This is, of course, just a quick look at what you can do with XSL Formatting Objects. If you take its power and combine it with both the ability to generate standard files such as PDFs and the fact that you can easily generate XSL-FO with XSL Transformations, you have an essential tool in your presentational toolbox.