In your bootstrap, you need to call something like
PHP Code:
$viewRenderer = Zend_Controller_Action_HelperBroker::getStaticHelper('viewRenderer');
$viewRenderer->initView();
$view = $viewRenderer->view;
$view->docType('HTML4_STRICT');
The last line tells helpers whether to produce HTML or XHTML markup.
You need to set the appropriate XHTML doctype to produce tags that end with />