I think, it's rather common task to make one element look-and-feel like other one. Sometimes I spend time comparing long lists of css properties and mess of css rules to find difference, which makes one element to look one pixel different than another. I think it might be usefule to be able easily build list of css properties with values, which values are different between two elements.
I have some ideas about how to do this in the case of one element changing, but nothing concrete to commit. In the case of unrelated elements, it seems pretty hopeless that we could definitively say anything about why.
<rdar://problem/19107209>