Make a comment more precise about derived properties.
Cascade declarations in reverse order, skipping those already seen

Previously we processed them in forward order, latter dcelarations for
the same property overriding any earlier one, making the work of
converting the earlier ones to a computed value redundant.

Maintaining a bit field of "seen" properties will also help fixing
#2288.
Parse declarations in reverse order, skip those that would be overridden.
Set the border-*-width computed values to 0 as appropriate. Fix #2288
Fix computed value of 'display' when no declaration applies.
auto merge of #2345 : SimonSapin/servo/reverse-declarations, r=pcwalton

… and deal with properties whose initial value can be affected at computed-value time.

r? @pcwalton