zend表单向Htmltag添加一个类

How can I go about adding a class to Html tag, I tried all different kinds of decorators methods but still can't come to a solution, here my preview html code Im trying to add a class to the li tag

<div class="col-md-12 field-box">
<dt id="firstname-label">
<dd id="firstname-element">
<input id="firstname" type="text" 1="Htmltag Array" 0="col-md-9 form-control" value="" name="firstname">
<ul class="errors">
<li>Value is required and can't be empty</li>
</ul>
</dd>
</div>

solutions tried

->addDecorator(array('row' => 'HtmlTag'), array('tag' => 'div','class' => 'col-md-12 field-box'), array('HtmlTag',array('tag' => 'li', 'class' =>'kopano')))

$firstName->setAttribs(array('class' => 'col-md-9 form-control',array('Htmltag', array('tag' => 'dd', 'class' =>'kopano'))));

$firstName->setAttribs(array('class' => 'col-md-9 form-control'),array('Htmltag', array('tag' => 'dd', 'class' =>'kopano')));

THANKS IN ADVANCE

If you want set the class 'kopano' to the HtmlTag of element $firstName try:

    $deco_html_tag = $firstName->getDecorator('HtmlTag');
    $deco_html_tag->setOption('class', 'kopano');

Obviously, this is not possible without creating Helper.
In Zend_View_Helper_FormErrors file the li tags are coded directly <li>

$deco_error_tag = $firstName->getDecorator('Errors');
$deco_error_tag->setOption('class', 'kopano');

But you can put in the ul tag:

And with Javascript, access to children ul tag