XML in a Nutshell, 3rd Edition

Processing Instructions

XML parsers are required to provide client applications access to XML processing instructions. Processing instructions provide a mechanism for document authors to communicate with XML-aware applications behind the scenes in a way that doesn’t interfere with the content of the document. DTD and schema validation both ignore processing instructions, making it possible to use them anywhere in a document structure without changing the DTD or schema. The processing instruction’s most widely recognized application is its ability to embed stylesheet references inside XML documents. The following XML fragment shows a stylesheet reference:

<?xml-stylesheet type="text/css" href="test.css"?>

An XML-aware application, such as Internet Explorer 6.0, would be capable of recognizing the XML author’s intention to display the document using the test.css stylesheet. This processing instruction can also be used to link to XSLT stylesheets or other kinds of stylesheets not yet developed, although the client application needs to understand how to process them to make this work. Applications that do not understand the processing instructions can still parse and use the information in the XML document while ignoring the unfamiliar processing instruction.

The furniture example from Chapter 21 (see Figure 21-1) gives a hypothetical application of processing instructions. A processing instruction in the bookcase.xml file signals the furniture example’s processor to verify the parts ...