diff --git a/components/style/properties/declaration_block.rs b/components/style/properties/declaration_block.rs
index 0d5ba518935..29277eef1d8 100644
--- a/components/style/properties/declaration_block.rs
+++ b/components/style/properties/declaration_block.rs
@@ -47,8 +47,12 @@ pub enum DeclarationPushMode {
     /// importance, the new declaration will be discarded. Otherwise, it will
     /// be appended to the end of the declaration block.
     Parsing,
+    /// In this mode, if there is an existing declaration of the same property,
+    /// the value is updated in-place. Otherwise it's appended. This is one
+    /// possible behavior of CSSOM.
+    Update,
     /// In this mode, the new declaration is always pushed to the end of the
-    /// declaration block. This is for CSSOM.
+    /// declaration block. This is another possible behavior of CSSOM.
     Append,
 }
 
@@ -414,10 +418,49 @@ impl PropertyDeclarationBlock {
         }
     }
 
+    /// Returns whether the property is definitely new for this declaration
+    /// block. It returns true when the declaration is a non-custom longhand
+    /// and it doesn't exist in the block, and returns false otherwise.
+    #[inline]
+    fn is_definitely_new(&self, decl: &PropertyDeclaration) -> bool {
+        match decl.id() {
+            PropertyDeclarationId::Longhand(id) => !self.longhands.contains(id),
+            PropertyDeclarationId::Custom(..) => false,
+        }
+    }
+
+    /// Returns whether calling extend with `DeclarationPushMode::Update`
+    /// will cause this declaration block to change.
+    pub fn will_change_in_update_mode(
+        &self,
+        source_declarations: &SourcePropertyDeclaration,
+        importance: Importance,
+    ) -> bool {
+        // XXX The type of parameter seems to be necessary because otherwise
+        // the compiler complains about `decl` not living long enough in the
+        // all_shorthand expression. Why?
+        let needs_update = |decl: &_| {
+            if self.is_definitely_new(decl) {
+                return true;
+            }
+            self.declarations.iter().enumerate()
+                .find(|&(_, ref slot)| slot.id() == decl.id())
+                .map_or(true, |(i, slot)| {
+                    let important = self.declarations_importance[i];
+                    *slot != *decl || important != importance.important()
+                })
+        };
+        source_declarations.declarations.iter().any(&needs_update) ||
+            source_declarations.all_shorthand.declarations().any(|decl| needs_update(&decl))
+    }
+
     /// Adds or overrides the declaration for a given property in this block.
     ///
     /// See the documentation of `push` to see what impact `source` has when the
     /// property is already there.
+    ///
+    /// When calling with `DeclarationPushMode::Update`, this should not change
+    /// anything if `will_change_in_update_mode` returns false.
     pub fn extend(
         &mut self,
         mut drain: SourcePropertyDeclarationDrain,
@@ -473,16 +516,7 @@ impl PropertyDeclarationBlock {
         importance: Importance,
         mode: DeclarationPushMode,
     ) -> bool {
-        let longhand_id = match declaration.id() {
-            PropertyDeclarationId::Longhand(id) => Some(id),
-            PropertyDeclarationId::Custom(..) => None,
-        };
-
-        let definitely_new = longhand_id.map_or(false, |id| {
-            !self.longhands.contains(id)
-        });
-
-        if !definitely_new {
+        if !self.is_definitely_new(&declaration) {
             let mut index_to_remove = None;
             for (i, slot) in self.declarations.iter_mut().enumerate() {
                 if slot.id() != declaration.id() {
@@ -514,6 +548,15 @@ impl PropertyDeclarationBlock {
                         }
                     }
                 }
+                if matches!(mode, DeclarationPushMode::Update) {
+                    let important = self.declarations_importance[i];
+                    if *slot == declaration && important == importance.important() {
+                        return false;
+                    }
+                    *slot = declaration;
+                    self.declarations_importance.set(i, importance.important());
+                    return true;
+                }
 
                 index_to_remove = Some(i);
                 break;
@@ -528,7 +571,7 @@ impl PropertyDeclarationBlock {
             }
         }
 
-        if let Some(id) = longhand_id {
+        if let PropertyDeclarationId::Longhand(id) = declaration.id() {
             self.longhands.insert(id);
         }
         self.declarations.push(declaration);