<p>The Date Popup calendar includes some css for IE6 that breaks css validation. Since IE 6 is now superceded by IE 7 and IE 8, the special css for IE 6 has been removed from the regular css used by the Date Popup. If you find you need that css after all, you can add it back in your theme. Look at the way the Garland theme adds special IE-only css in in its page.tpl.php file. The css you need is:</p>