If you have an element, e.g. a submit button, you can create it with this definition:
$button = array( 'type' => 'submit', 'name' => 'buttonname', 'cssclass' => 'mw-ui-button mw-ui-destructive', )
If you now create a form with a div layout or a vform the class mentioned in cssclass is added to the wrapper div and the button itself, which makes the whole fiv a mw-ui-button (any following element hasn't a margin e.g.). That seems totally false, so, for a div layout, the class shouldn't be added to the wrapper div?