style: Introduce the concept of legacy shorthands.

We need this because there's a weird mapping between these properties' values
('always' maps to 'page').

See https://drafts.csswg.org/css-cascade-4/#legacy-shorthand.

Differential Revision: https://phabricator.services.mozilla.com/D12213

components/style/properties/declaration_block.rs

@@ -917,6 +917,10 @@ impl PropertyDeclarationBlock {
                 }
                 already_serialized.insert(shorthand.into());
 
+                if shorthand.is_legacy_shorthand() {
+                    continue;
+                }
+
                 // Substep 2 & 3
                 let mut current_longhands = SmallVec::<[_; 10]>::new();
                 let mut important_count = 0;