[cssom-view] Why is body's offsetParent null ?

The current spec and user agents I have tested - IE7, FF3b5, Safari 3.1, Opera 9.5b1 -  set body's offsetParent property to null even though a) body has offset properties for which b) there is a reference element - html - with box properties of its own.

My question is : what are the reasons for this beyond compatibility with IE legacy ?

Notes:
- In standards mode, IE8 Beta 1 currently sets body's offsetParent to html; html's offsetParent is null.
- IE quirks mode may return html as the offsetParent of a child element of body.

Thanks,
Sylvain