What this fix does is adds the ALT data for the captcha image in the leavemessage.tpl, here is the fix:
Go to: styles/original/templates/leavemessage.tpl
Find: <td class="text"><img src="captcha.php"/></td>
Replace with: <td class="text"><img src="captcha.php" alt="captcha image" /></td>
That's all there is to it. I haven't found any other issues with Mibew and validation other than this. By the way this fix is for the "Original" style. That is the only style I use but I am sure it is the same sort of fix for other styles. I hope this helps!