XML Reference Guide

Mar 14, 2003

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Cascading Style Sheets, or CSS, are typically used in the HTML environment, but they are also a great way to control the presentation of XML data. Unlike XSL-FO, which also controls presentation, CSS provides a way to specify presentation outside the environment of the XML itself, so it's a great tool for standardizing the look and feel of information. But let's start at the beginning.

If I were building an HTML page, I could easily specify a section that includes a headline and some other text:

<html>
<head><title>Chaos Magnet</title></head>
<body>
   <div>
      <h1>Chaos Magnet</h1>      <p>Murphy never had it like this.</p>
   </div>
</body>
</html>

Figure CSS1

Because this is standard HTML, the browser knows that each element has certain properties. For example, both h1 and p are block elements, meaning that each is displayed on its own line, even though the HTML shows them on the same line. Also, h1 has inherent font-size and font-weight properties that tell the browser to display it as large, bold text.

In addition to these inherent properties, we can set additional properties. For example, we can add a border to the div and change the font and color of the heading:

<html>
<head><title>Chaos Magnet</title></head>
<body>
   <div style="border: 1px solid black; padding: 10px;">
      <h1 style="color: red; font-family: arial;">Chaos Magnet</h1>
      <p style="font-family: arial;">Murphy never had it like this.</p>
   </div>
</body>
</html>

Figure CSS2

Of course, that can get kind of messy, particularly if we have more than one of any particular tag. Can you imagine trying to change the look-and-feel of a website when you have to manually change the style attribute on every single element? Fortunately, you can, instead, specify style information separately:

<html>
<head><title>Chaos Magnet</title>

   <style type="text/css">
      *     { font-family: arial }
      h1    { color: red }
      div   { border: 1px solid black;
              padding: 10px; }
   </style>

</head>
<body>
   <div>
      <h1>Chaos Magnet</h1>
      <p>Murphy never had it like this.</p>
   </div>
</body>
</html>

Each style consists of two pieces of information: the selector and the directive. The selector tells the browser what information should be effected by the change. For example, only h1 elements should be red, so we used the h1 selector. On the other hand, we wanted all text to use the Arial font, so we used *, the so-called "universal selector."

The information in the brackets after the selector is one or more directives. These directives consist of a property to set (such as the color) and the value to set it to (such as red). Note that elements can be effected by more than one rule. The h1 element is red, but it's also Arial.

So what does all of this have to do with XML? Well, for one thing, we can also use CSS to control the presentation of straight XML in the browser. This means that instead of using HTML-specific elements, we could create a document that's more descriptive:

<document>
   <blogtitle>Chaos Magnet</blogtitle>
   <description>Murphy never had it like this.</description>
</document>

If we try to show this document in the browser "as-is," the browser doesn't have any information on presentation, because these are not recognized elements. Most modern browsers just display some version of the document tree:

Figure CSS3

To add style information, we can use a processing instruction to specify a style sheet, just as we would normally add an XSLT style sheet:

<?xml-stylesheet type="text/css" href="blogstyles.css"?>
<document>
   <blogtitle>Chaos Magnet</blogtitle>
   <description>Murphy never had it like this.</description>
</document>

Notice that the type is text/css rather than text/xsl, as it is with an XSLT style sheet. From here, we can create a style sheet that duplicates our original HTML page:

*                 { font-family: arial }
blogtitle         { display: block;
                    color: red;
                    font-size: 24pt;
                    font-weight: bold;
                    margin-top: 25px;
                    margin-bottom: 25px; }
blogdescription   { display: block; }
document          { display: block;
                    border: 1px solid black;
                    padding: 10px;
                    margin: 10px; }

Notice that we had to add more information than we did for the HTML tags, because there are no pre-determined properties that the browser recognizes.

That covers the basics. For more information on the various types of selectors (such as attributes, siblings, classes, and so on) and the dozens of available properties, see the official documentation or these InformIT articles.

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