Currently the close button is an anchor element. From overlay.tpl.php
<div id="overlay-close-wrapper">
<a id="overlay-close" href="#" class="overlay-close"><span class="element-invisible"><?php print t('Close overlay'); ?></span></a>
</div>The anchor element should be updated to a button so that it is made accessible to screen reader users.
Linked from #890288: Improve Overlay accessibility by using modal dialogs.