The ProcessingInstruction interface represents a processing instruction; that is, a Node which embeds an instruction targeting a specific application but that can be ignored by any other applications which don't recognize the instruction.
Warning: ProcessingInstruction nodes are only supported in XML documents, not in HTML documents. In these, a process instruction will be considered as a comment and be represented as a Comment object in the tree.
A processing instruction may be different than the XML declaration.
Note: User-defined processing instructions cannot begin with "xml", as xml-prefixed processing-instruction target names are reserved by the XML specification for particular, standard uses (see, for example, <?xml-stylesheet ?>.
For example:
html
<?xml version="1.0"?>
is a processing instruction whose target is xml.