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Processing Instruction

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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.



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"?>

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 uses processing instructions.[8]


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