Use class "element-invisible" instead of style "display:none"

Please use the class "element-invisible" instead of hiding the label to improve the usability for screenreaders:

Change form_placeholder.js at line 57

             if (!$textfield.attr('placeholder')) {
             $textfield.attr('placeholder', $.trim($label.text()));
-            $label.hide();
+            $label.addClass('element-invisible');
           }
 
           // Fallback support for older browsers.

(Sorry – i'm not yet able to create a real working patch …)