A processing instruction (PI) is an SGML and XML node type, which may occur anywhere in a 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]
The XML Declaration at the beginning of an XML document (shown below) is another example of a processing instruction,[5] however it may not technically be considered one.[6]
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.[7] 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.[8]
A draft specification for Robots exclusion standard rules inside XML documents uses processing instructions.[9]