HTML5 allows class attributes on the root element. The CSS signatures (classes like "mediawiki ns-* page-* skin-*" etc.) that are currently applied to the body element could therefore be moved to the root element.
The advantage is that these signatures can then be used to style even the root element, which is currently impossible (see http://annevankesteren.nl/2004/08/signatures for the rationale; the post is about ID attributes, but still applicable).
This already works in every browser that allows styling the root element (IE >= 6 and all others) and even in IE 5.5 (which doesn't allow styling the root element, but still applies rules with selectors that start at the root element). Testcase attached.
Note, however, that this will break existing rules with unnecessarily specific selectors ("body.mediawiki" instead of just ".mediawiki"). It may therefore be wise to add the classes to both elements for a while.