[css3-page] "Tree order" (default stacking order) of page-margin boxes

Hi,

 From css3-page §4.1:
> The default painting order, or CSS2.1 Appendix E "tree order", of
> page-margin boxes with respect to each other is not specified.

I am not aware of a reason for leaving this unspecified, other than we 
haven’t found an order that makes sense independently of the writing 
mode/direction.

I suggest picking an arbitrary order: top-left-corner is first (below), 
then go clockwise for the remaining 15 boxes.

This is only the default order: authors can change it with z-index. It 
is inspired from the 'margin' and 'border-radius' shorthands.

-- 
Simon Sapin