The recent update, 3.1.6 should generate an output valid for HTML5
However, this breaks validity for XHTML 1.0 Transitional...
Please insert the
type attribute for the
style tag, something like
<style type="text/plain" id="syntaxhighlighteranchor"></style>
This is more accurate HTML5, also valid for XHTML (didn't test all versions, though)