diff --git a/dom/animation/KeyframeEffectReadOnly.cpp b/dom/animation/KeyframeEffectReadOnly.cpp
index 9dc4d735b9fc..d413438d6ed2 100644
--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -430,6 +430,12 @@ KeyframeEffectReadOnly::EnsureBaseStylesForCompositor(
       continue;
     }
 
+    // We only call SetNeedsBaseStyle after calling GetBaseStyle so if
+    // NeedsBaseStyle is true, the base style should be already filled-in.
+    if (NeedsBaseStyle(property.mProperty)) {
+      continue;
+    }
+
     for (const AnimationPropertySegment& segment : property.mSegments) {
       if (segment.mFromComposite == dom::CompositeOperation::Replace &&
           segment.mToComposite == dom::CompositeOperation::Replace) {
@@ -473,18 +479,15 @@ KeyframeEffectReadOnly::ComposeStyle(
   // time so we shouldn't animate.
   if (computedTiming.mProgress.IsNull()) {
     // If we are not in-effect, this effect might still be sent to the
-    // compositor and later become in-effect (e.g. if it is in the delay phase).
+    // compositor and later become in-effect (e.g. if it is in the delay phase,
+    // or, if it is in the end delay phase but with a negative playback rate).
     // In that case, we might need the base style in order to perform
     // additive/accumulative animation on the compositor.
 
-    // In case of properties that can be run on the compositor, we need the base
-    // styles for such properties because those animation will be sent to
-    // compositor while they are in delay phase so that we can composite this
-    // animation on the compositor once the animation is out of the delay phase
-    // on the compositor.
-    if (computedTiming.mPhase == ComputedTiming::AnimationPhase::Before) {
-      EnsureBaseStylesForCompositor(aPropertiesToSkip);
-    }
+    // Note, however, that we don't actually send animations with a negative
+    // playback rate in their end delay phase to the compositor at this stage
+    // (bug 1330498).
+    EnsureBaseStylesForCompositor(aPropertiesToSkip);
     return;
   }
 
@@ -591,6 +594,14 @@ KeyframeEffectReadOnly::ComposeStyle(
       aStyleRule->AddValue(prop.mProperty, Move(toValue));
     }
   }
+
+  // For properties that can be run on the compositor, we may need to prepare
+  // base styles to send to the compositor even if the current processing
+  // segment for properties does not have either an additive or accumulative
+  // composite mode, and even if the animation is not in-effect. That's because
+  // the animation may later progress to a segment which has an additive or
+  // accumulative composite on the compositor mode.
+  EnsureBaseStylesForCompositor(aPropertiesToSkip);
 }
 
 bool