And one more thought.
This one from CSS practical implementation point of view:
We should try to minimise as much as possible number of attributes in style
structure.
To reduce consumption of memory occupied and processor power needed to
manage styles (e.g. inheritance).
Of course if we want to keep UA as a "thin client". If this definition is
still applicable...

Andrew Fedoniouk.
MS, Physics and Applied Mathematics.
Terra Informatica Software Design.
http://terrainformatica.com 
Richmond, British Columbia, Canada.
+1(604) 244-1074