Box model properties and the root element

Should not the root element of an unstyled XML document have zero
margins in the absence of the possibility of a conceptual or concrete UA
style sheet (i.e., because there is no elements that the UA can style)? 

Clearly :root {display: block; margin: 5px /* Or whatever */} is
required for current UA behaviour, but this prevents a difficulty: it is
also necessary that UAs specify margins not on HTML (the root element)
but rather on BODY for compatibility with current documents (i.e., since
current practice is to specify margins on BODY, which override those of
the user agents whereas to specify them on HTML (:root) would be mean
that those specified by the document author would be in addition to
those on BODY). 

So what then is the solution to this logical difficulty?

There are, as I see it, three solutions:

A. that :root should match BODY for HTML documents for reasons of
compatibility with 'legacy' content
B. that different conceptual style sheets should apply to documents with
an HTML dtd than other dtds
C. that :root should not apply to HTML documents at all

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