A Processing Instruction (PI) is an SGML and XML node type, which may occur anywhere in the document, intended to carry instructions to the application.[1][2]
Processing instructions are exposed in the Document Object Model as Node.PROCESSING_INSTRUCTION_NODE, and they can be used in XPath and XQuery with the 'processing-instruction()' command.
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An SGML processing instruction is enclosed within '<? and '>'.[3]
An XML processing instruction is enclosed within '<?' and '?>', and contains a target and optionally some content, which is the node value, that cannot contain the sequence '?>'.[4]
<?PITarget PIContent?>
The XML Declaration at the beginning of an XML document (shown below) is not a processing instruction, however its similar syntax has often resulted in it being referred to as a processing instruction.[5]
<?xml version="1.0" encoding="UTF-8" ?>
The most common use of a processing instruction is to request the XML document be rendered using a stylesheet using the 'xml-stylesheet' target, which was standardized in 1999.[6] It can be used for both XSLT and CSS stylesheets.
<?xml-stylesheet type="text/xsl" href="style.xsl"?>
<?xml-stylesheet type="text/css" href="style.css"?>
Another use is the programming language PHP, which can be embedded within an (X)HTML document as shown in the following example.[5]
<?php echo $a; ?>
The DocBook XSLT stylesheets understand a number of processing instructions to override the default behaviour.[7]
A draft specification for Robots exclusion standard rules inside XML documents.[8]