Use span elements instead of div inside button element

According to the HTML5 spec, the button element must have phrasing content, i.e. div is not allowed inside button. [http://www.w3.org/TR/html5/forms.html#the-button-element]

I’ve changed the 2 div elements inside button to span elements. Changes to the stylesheet ('display: block') are not necessary because 'position: absolute' is applied to both elements already.