diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 0ce4a45d175..ffac86e974f 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -37,6 +37,7 @@ use gecko_bindings::bindings::Gecko_ElementHasAnimations;
 use gecko_bindings::bindings::Gecko_ElementHasCSSAnimations;
 use gecko_bindings::bindings::Gecko_ElementHasCSSTransitions;
 use gecko_bindings::bindings::Gecko_GetActiveLinkAttrDeclarationBlock;
+use gecko_bindings::bindings::Gecko_GetAnimationEffectCount;
 use gecko_bindings::bindings::Gecko_GetAnimationRule;
 use gecko_bindings::bindings::Gecko_GetExtraContentStyleDeclarations;
 use gecko_bindings::bindings::Gecko_GetHTMLPresentationAttrDeclarationBlock;
@@ -948,8 +949,16 @@ fn get_animation_rule(
     cascade_level: CascadeLevel,
 ) -> Option<Arc<Locked<PropertyDeclarationBlock>>> {
     use gecko_bindings::sugar::ownership::HasSimpleFFI;
+    use properties::longhands::ANIMATABLE_PROPERTY_COUNT;
+
+    // There's a very rough correlation between the number of effects
+    // (animations) on an element and the number of properties it is likely to
+    // animate, so we use that as an initial guess for the size of the
+    // AnimationValueMap in order to reduce the number of re-allocations needed.
+    let effect_count = unsafe { Gecko_GetAnimationEffectCount(element.0) };
     // Also, we should try to reuse the PDB, to avoid creating extra rule nodes.
-    let mut animation_values = AnimationValueMap::default();
+    let mut animation_values = AnimationValueMap::with_capacity_and_hasher(
+        effect_count.min(ANIMATABLE_PROPERTY_COUNT), Default::default());
     if unsafe {
         Gecko_GetAnimationRule(
             element.0,