Make OpenSeadragon W3C compliant.

I just discovered this firefox plugin: http://users.skynet.be/mgueury/mozilla/

So here is what was needed to make OSD W3C compliant:
* add alt attributes to the img elements
* change button element to div because a button can not have a child element
* change fieldgroup to div because a fieldgroup can not have a child div
* change span to div for the same reason
* change form to div because you can't have a form in a form.