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.
Contents
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.
Syntax
An SGML processing instruction is enclosed within <? and >.
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 ?>.
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.
Examples
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. It can be used for both XSLT and CSS stylesheets.
The DocBook XSLT stylesheets understand a number of processing instructions to override the default behaviour.
A draft specification for Robots exclusion standard rules inside XML documents uses processing instructions.