[css-logical-props] resolving against parent's writing mode properties

I don’t understand the reasoning behind resolving the margin-* and
padding-* logical properties against the writing mode properties of the
parent, rather than the element itself.  Can someone explain, and
perhaps add that explanation to the spec as a note?

Also, the spec should define what happens when you use one of these
logical properties on the root element.

-- 
Cameron McCormack ≝ http://mcc.id.au/