The latest version of JComments fails WCAG validation because there is no label for the captcha form field (and is therefore technically illegal in most developed countries).
The fix is to change the line:
<input class="captcha" id="comments-form-captcha" type="text" name="captcha_refid" value="" size="5" tabindex="6" /><br />
to:
<input class="captcha" id="comments-form-captcha" type="text" name="captcha_refid" value="" size="5" tabindex="6" />
<label for="comments-form-captcha"><?php echo JText::_('FORM_CAPTCHA'); ?></label><br />
in com_jcomments/tpl/default/tpl_form.php (line 125).
OpenGlobal