Hi - just checked again - the footer code you have in your mod - causes the page to fail validation - if it is replaced with the original code - like this:
define('FOOTER_TEXT_BODY', 'Copyright &copy; ' . date('Y') . ' <a href="' . zen_href_link(FILENAME_DEFAULT) . '" target="_blank">' . STORE_NAME . '</a>. Powered by <a href="http://www.zen-cart.com" target="_blank">Zen Cart</a>');
Now passes validation again....