Whoops, looks like it's actually supposed to be stylized "HTML5" instead of "HTML 5".
Change to be made in lib/tpl/dokuwiki/tpl_footer.php:
23: <a href="http://validator.w3.org/check/referer" title="Valid HTML5" <?php echo $target?>><img
24: src="<?php echo tpl_basedir(); ?>images/button-html5.png" width="80" height="15" alt="Valid HTML5" /></a>