When you render a Zend_Form, the elements will render to HTML compliance rather than XHTML compliance, even if you have < ?php echo $this->doctype('XHTML1_STRICT');?> at the top of your layout script. Practically, this means that all the input elements do not end in "/>". To resolve this, you need to call the doctype() view helper prior to rendering your form.

He includes an example of the doctype call (on a view object) to tell the script to output in the "XHTML1_TRANSITIONAL" format.