Processing Instructions are like comments accept they are meant for application-specific processing.

Syntax:

[16]    PI        ::=    '<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'
[17]    PITarget  ::=    Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))

That is, a processing instruction contains text except that '?>' is not allowed in the content and 'xml' is not allow in the name.

Example:

<?insert 5/10/03?>

Not allowed:

<?very ?> bad ?>

But the W3C can break the rules:

<?xml-stylesheet href="mystyle.xsl"?>