We should look into not serving OOUI styles for IE 6 and 7. That might be a better way of providing grade C support (T124905) than trying to make the style look right.
OOUI PHP is mostly using fairly semantic HTML for the widgets, and the demo page looks almost good when stripped of its styles:
We could probably make it pretty good by switching some <div>s to <span>s and maybe adding some <p>s in strategic locations. That would surely be a good thing to do anyway.