document.body works for XHTML documents in Firefox, but not in WebKit.
Created attachment 12959 [details]
We should consider supporting all HTMLDocument properties and methods on all XML documents (because any XML document could contain XHTML).
See also: bug 14213.
This also accounts some TypeError's in layout tests under dom/xhtml.
(In reply to comment #1)
> Created an attachment (id=12959) 
> test case
Fixed in revision 26078 (feature branch).