This will also simplify the implementation. Instead one doubles a factor which is associated with the attribute. Then, the factor is multiplied in with whatever size the attribute has been associated with. Having two different names for the same property is confusing. The grouping of elements is there to make the syntax less verbose.
Rules allow no expressions.
There are two issues here. A good principle when determining the presentation is that it should take place as close as possible to the user, both spatially and temporally, since one there will have more information available. By enclosing logic in the style sheets, final decisions can be made closer to the user. Cached style sheets will be more general and can live longer. Also, this scheme requires no changes on the server side.