diff --git a/components/style/gecko/generated/bindings.rs b/components/style/gecko/generated/bindings.rs
index 69fbfda33f71..c77274bbf39d 100644
--- a/components/style/gecko/generated/bindings.rs
+++ b/components/style/gecko/generated/bindings.rs
@@ -2562,7 +2562,9 @@ extern "C" {
     pub fn Servo_DeclarationBlock_SerializeOneValue(declarations:
                                                         RawServoDeclarationBlockBorrowed,
                                                     property: nsCSSPropertyID,
-                                                    buffer: *mut nsAString);
+                                                    buffer: *mut nsAString,
+                                                    computed_values:
+                                                        ServoStyleContextBorrowedOrNull);
 }
 extern "C" {
     pub fn Servo_DeclarationBlock_Count(declarations:
diff --git a/components/style/properties/declaration_block.rs b/components/style/properties/declaration_block.rs
index 625ef96465ee..07bc5eeab1dd 100644
--- a/components/style/properties/declaration_block.rs
+++ b/components/style/properties/declaration_block.rs
@@ -518,13 +518,35 @@ impl PropertyDeclarationBlock {
     }
 
     /// Take a declaration block known to contain a single property and serialize it.
-    pub fn single_value_to_css<W>(&self, property: &PropertyId, dest: &mut W) -> fmt::Result
-        where W: fmt::Write,
+    pub fn single_value_to_css<W>(
+        &self,
+        property: &PropertyId,
+        dest: &mut W,
+        computed_values: Option<&ComputedValues>,
+    ) -> fmt::Result
+    where
+        W: fmt::Write,
     {
         match property.as_shorthand() {
             Err(_longhand_or_custom) => {
                 if self.declarations.len() == 1 {
-                    self.declarations[0].0.to_css(dest)
+                    let declaration = &self.declarations[0].0;
+                    // If we have a longhand declaration with variables, those variables will be
+                    // stored as unparsed values. As a temporary measure to produce sensible results
+                    // in Gecko's getKeyframes() implementation for CSS animations, if
+                    // |computed_values| is supplied, we use it to expand such variable
+                    // declarations. This will be fixed properly in Gecko bug 1391537.
+                    match (declaration, computed_values) {
+                        (&PropertyDeclaration::WithVariables(id, ref unparsed),
+                         Some(ref computed_values)) => unparsed
+                            .substitute_variables(
+                                id,
+                                &computed_values.custom_properties(),
+                                QuirksMode::NoQuirks,
+                            )
+                            .to_css(dest),
+                        (ref d, _) => d.to_css(dest),
+                    }
                 } else {
                     Err(fmt::Error)
                 }
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index 4a1d04d519ca..c0b778555651 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -564,7 +564,8 @@ pub extern "C" fn Servo_AnimationValue_Serialize(value: RawServoAnimationValueBo
     let uncomputed_value = AnimationValue::as_arc(&value).uncompute();
     let mut string = String::new();
     let rv = PropertyDeclarationBlock::with_one(uncomputed_value, Importance::Normal)
-        .single_value_to_css(&get_property_id_from_nscsspropertyid!(property, ()), &mut string);
+        .single_value_to_css(&get_property_id_from_nscsspropertyid!(property, ()), &mut string,
+                             None);
     debug_assert!(rv.is_ok());
 
     let buffer = unsafe { buffer.as_mut().unwrap() };
@@ -2142,12 +2143,13 @@ pub extern "C" fn Servo_DeclarationBlock_GetCssText(declarations: RawServoDeclar
 #[no_mangle]
 pub extern "C" fn Servo_DeclarationBlock_SerializeOneValue(
     declarations: RawServoDeclarationBlockBorrowed,
-    property_id: nsCSSPropertyID, buffer: *mut nsAString)
+    property_id: nsCSSPropertyID, buffer: *mut nsAString,
+    computed_values: ServoStyleContextBorrowedOrNull)
 {
     let property_id = get_property_id_from_nscsspropertyid!(property_id, ());
     read_locked_arc(declarations, |decls: &PropertyDeclarationBlock| {
         let mut string = String::new();
-        let rv = decls.single_value_to_css(&property_id, &mut string);
+        let rv = decls.single_value_to_css(&property_id, &mut string, computed_values);
         debug_assert!(rv.is_ok());
 
         let buffer = unsafe { buffer.as_mut().unwrap() };