HTML validation fails because of the tag <meta id="syntaxhighlighteranchor" [...] />
Your comments in an earlier post stated that it was needed as a way to load CSS, and concluded that you would be trying to fix having to use it. (v of the plug in was 3.0.1)
The tag is still there in v3.1.3
I was able to validate by simply commenting out the
echo '<meta id="syntaxhighlighteranchor" name="syntaxhighlighter-version" content="' . esc_attr( $this->pluginver ) . '" />' . "\n";
line, but I really hate to edit plugins since the next update will remove my hack.