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7.2. Direct Display of XML in Browsers

Ultimately, one hopes that browsers will be able to display not just XHTML documents but any XML document as well. Since it's too much to ask that browsers provide semantics for all XML applications both current and yet-to-be-invented, stylesheets will be attached to each document to provide instructions about how each element will be rendered.

The current major stylesheet languages are:

Eventually, there will be still more versions of these, including at least CSS3 and XSLT 2.0. However, let's begin by looking at how and how well existing style languages are supported by existing browsers.

7.2.1. The xml-stylesheet Processing Instruction

The stylesheet associated with a document is indicated by an xml-stylesheet processing instruction in the document's prolog, which comes after the XML declaration but before the root element start-tag. This processing instruction uses pseudoattributes to describe the stylesheet (that is, they look like attributes but are not attributes because xml-stylesheet is a processing instruction and not an element).

7.2.2. Internet Explorer

Microsoft Internet Explorer 4.0 (IE4) includes an XML parser that can be accessed from VBScript or JavaScript. This is used internally to support channels and the Active Desktop. Your own JavaScript and VBScript programs can use this parser to read XML data and insert it into the web page. However, anything more straightforward, like simply displaying a page of XML from a specified URL, is beyond IE4's capabilities. Furthermore, IE4 doesn't understand any stylesheet language when applied to XML.

Internet Explorer 5 (IE5) and 5.5 (IE 5.5) do understand XML, though their parser is more than a little buggy; it rejects a number of documents it shouldn't reject, most embarrassingly the XML 1.0 specification itself. Internet Explorer 6 (IE6) has improved XML support somewhat, but is still not completely conformant.

IE5 and later can directly display XML files, with or without an associated stylesheet. If no stylesheet is provided, then IE5 uses a default, built-in XSLT stylesheet that displays the tree structure of the XML document along with a little DHTML to allow the user to collapse and expand nodes in the tree. Figure 7-1 shows IE5 displaying Example 6-1 from the last chapter.

Figure 7-1

Figure 7-1. A document that uses IE5's built-in stylesheet

IE5 also supports parts of CSS Level 1 and a little of CSS Level 2. However, the support is spotty and inconsistent. Even some aspects of CSS that work for HTML documents fail when applied to XML documents. IE 5.5 and IE6 slightly improve coverage of CSS, but don't support all CSS properties and selectors. In fact, many CSS features that work in IE6 for HTML still don't work when applied to XML documents.

IE5 and IE 5.5 support their own custom version of XSLT, based on a very early working draft of the XSLT specification. They do not support XSLT 1.0. You can tell the difference by looking at the namespace of the stylesheet. A stylesheet written for IE5 uses the http://www.w3.org/TR/WD-xsl namespace, whereas a stylesheet designed for standard-compliant XSLT processors uses the http://www.w3.org/1999/XSL/Transform namespace. Despite superficial similarities, these two languages are not compatible. A stylesheet written for IE5 will not work with any other XSLT processor, and a stylesheet written using standard XSLT 1.0 will not work in IE5. IE6 supports both real XSLT and Microsoft's nonstandard dialect.

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