diff --git a/components/style/properties/declaration_block.rs b/components/style/properties/declaration_block.rs
index 0cc0aeca2fc..c332b4b07da 100644
--- a/components/style/properties/declaration_block.rs
+++ b/components/style/properties/declaration_block.rs
@@ -590,7 +590,7 @@ impl ToCss for PropertyDeclarationBlock {
         // Step 1 -> dest = result list
 
         // Step 2
-        let mut already_serialized = Vec::new();
+        let mut already_serialized = PropertyDeclarationIdSet::new();
 
         // Step 3
         for &(ref declaration, importance) in &*self.declarations {
@@ -598,7 +598,7 @@ impl ToCss for PropertyDeclarationBlock {
             let property = declaration.id();
 
             // Step 3.2
-            if already_serialized.contains(&property) {
+            if already_serialized.contains(property) {
                 continue;
             }
 
@@ -606,8 +606,10 @@ impl ToCss for PropertyDeclarationBlock {
             let shorthands = declaration.shorthands();
             if !shorthands.is_empty() {
                 // Step 3.3.1
+                //
+                // FIXME(emilio): All this looks terribly inefficient...
                 let mut longhands = self.declarations.iter()
-                    .filter(|d| !already_serialized.contains(&d.0.id()))
+                    .filter(|d| !already_serialized.contains(d.0.id()))
                     .collect::<Vec<_>>();
 
                 // Step 3.3.2
@@ -642,10 +644,10 @@ impl ToCss for PropertyDeclarationBlock {
                         }
                         // Substep 1:
                         //
-                         // Assuming that the PropertyDeclarationBlock contains no
-                         // duplicate entries, if the current_longhands length is
-                         // equal to the properties length, it means that the
-                         // properties that map to shorthand are present in longhands
+                        // Assuming that the PropertyDeclarationBlock contains no
+                        // duplicate entries, if the current_longhands length is
+                        // equal to the properties length, it means that the
+                        // properties that map to shorthand are present in longhands
                         if current_longhands.len() != properties.len() {
                             continue;
                         }
@@ -725,7 +727,7 @@ impl ToCss for PropertyDeclarationBlock {
 
                     for current_longhand in &current_longhands {
                         // Substep 9
-                        already_serialized.push(current_longhand.id());
+                        already_serialized.insert(current_longhand.id());
                         let index_to_remove = longhands.iter().position(|l| l.0 == **current_longhand);
                         if let Some(index) = index_to_remove {
                             // Substep 10
@@ -736,7 +738,7 @@ impl ToCss for PropertyDeclarationBlock {
             }
 
             // Step 3.3.4
-            if already_serialized.contains(&property) {
+            if already_serialized.contains(property) {
                 continue;
             }
 
@@ -756,7 +758,7 @@ impl ToCss for PropertyDeclarationBlock {
                 &mut is_first_serialization)?;
 
             // Step 3.3.8
-            already_serialized.push(property);
+            already_serialized.insert(property);
         }
 
         // Step 4