XML

Comments are delimited between 
Processing instructions allow a document to contain instructions to applications. The are of the form <? Instructions ?>.
CDATA sections are used to escape characters that would otherwise be considered XML markup. They are of the form <![CDATA[ datahere ]]>
If you just want to escape a character it is better to use the character entities,
All documents must begin with an XML declaration like <?xml version="1.0"?>