The prolog header is the following piece of code starting some XML documents:

<?xml version="y.x" encoding="zzzzz"?>

This requirement is explicitly defined in the XML specification:

Because each XML entity not accompanied by external encoding information and not in UTF-8 or UTF-16 encoding must begin with an XML encoding declaration, in which the first characters must be '<?xml', any conforming processor can detect, after two to four octets of input, which of the following cases apply.

Noncompliant Code Sample

<!-- Generated file -->
<?xml version="1.0" encoding="UTF-8"?>
<firstNode>
  content
</firstNode>

Compliant Solution

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated file -->
<firstNode>
  content
</firstNode>