--- a/js/src/devtools/rootAnalysis/analyzeHeapWrites.js
+++ b/js/src/devtools/rootAnalysis/analyzeHeapWrites.js
@@ -189,16 +189,18 @@ function treatAsSafeArgument(entry, varN
         ["Gecko_ClearPODTArray", "aArray", null],
         ["Gecko_ClearAndResizeStyleContents", "aContent", null],
         [/Gecko_ClearAndResizeCounter/, "aContent", null],
         [/Gecko_CopyCounter.*?From/, "aContent", null],
         [/Gecko_SetContentData/, "aContent", null],
         [/Gecko_EnsureStyle.*?ArrayLength/, "aArray", null],
         ["Gecko_AnimationAppendKeyframe", "aKeyframes", null],
         ["Gecko_GetOrCreateKeyframeAtStart", "aKeyframes", null],
+        ["Gecko_GetOrCreateInitialKeyframe", "aKeyframes", null],
+        ["Gecko_GetOrCreateFinalKeyframe", "aKeyframes", null],
         ["Gecko_SetStyleCoordCalcValue", null, null],
         ["Gecko_StyleClipPath_SetURLValue", "aClip", null],
         ["Gecko_nsStyleFilter_SetURLValue", "aEffects", null],
         ["Gecko_nsStyleSVGPaint_CopyFrom", "aDest", null],
         ["Gecko_nsStyleSVGPaint_SetURLValue", "aPaint", null],
         ["Gecko_nsStyleSVGPaint_Reset", "aPaint", null],
         ["Gecko_nsStyleSVG_SetDashArrayLength", "aSvg", null],
         ["Gecko_nsStyleSVG_CopyDashArray", "aDst", null],
@@ -371,16 +373,18 @@ function ignoreContents(entry)
         /nsCSSProps::LookupPropertyValue/,
         /nsCSSProps::ValueToKeyword/,
         /nsCSSKeywords::GetStringValue/,
 
         // The analysis can't cope with the indirection used for the objects
         // being initialized here.
         "Gecko_AnimationAppendKeyframe",
         "Gecko_GetOrCreateKeyframeAtStart",
+        "Gecko_GetOrCreateInitialKeyframe",
+        "Gecko_GetOrCreateFinalKeyframe",
         "Gecko_NewStyleQuoteValues",
         "Gecko_NewCSSValueSharedList",
         "Gecko_NewGridTemplateAreasValue",
         /nsCSSValue::SetCalcValue/,
         /CSSValueSerializeCalcOps::Append/,
         "Gecko_CSSValue_SetFunction",
         "Gecko_CSSValue_SetArray",
         "Gecko_EnsureMozBorderColors",

--- a/layout/style/ServoBindings.cpp
+++ b/layout/style/ServoBindings.cpp
@@ -1463,48 +1463,112 @@ Gecko_AnimationAppendKeyframe(RawGeckoKe
       aTimingFunction->mType != nsTimingFunction::Type::Linear) {
     keyframe->mTimingFunction.emplace();
     keyframe->mTimingFunction->Init(*aTimingFunction);
   }
 
   return keyframe;
 }
 
-Keyframe*
-Gecko_GetOrCreateKeyframeAtStart(nsTArray<Keyframe>* aKeyframes,
-                                 float aOffset,
-                                 const nsTimingFunction* aTimingFunction)
+enum class KeyframeSearchDirection {
+  Forwards,
+  Backwards,
+};
+
+enum class KeyframeInsertPosition {
+  Prepend,
+  LastForOffset,
+};
+
+static Keyframe*
+GetOrCreateKeyframe(nsTArray<Keyframe>* aKeyframes,
+                    float aOffset,
+                    const nsTimingFunction* aTimingFunction,
+                    KeyframeSearchDirection aSearchDirection,
+                    KeyframeInsertPosition aInsertPosition)
 {
   MOZ_ASSERT(aKeyframes, "The keyframe array should be valid");
   MOZ_ASSERT(aTimingFunction, "The timing function should be valid");
   MOZ_ASSERT(aOffset >= 0. && aOffset <= 1.,
              "The offset should be in the range of [0.0, 1.0]");
-  MOZ_ASSERT(aKeyframes->IsEmpty() ||
-             aKeyframes->ElementAt(0).mOffset.value() >= aOffset,
-             "The offset should be less than or equal to the first keyframe's "
-             "offset if there are exisiting keyframes");
 
   size_t keyframeIndex;
-  if (nsAnimationManager::FindMatchingKeyframe(*aKeyframes,
-                                               aOffset,
-                                               *aTimingFunction,
-                                               keyframeIndex)) {
-    return &(*aKeyframes)[keyframeIndex];
+  switch (aSearchDirection) {
+    case KeyframeSearchDirection::Forwards:
+      if (nsAnimationManager::FindMatchingKeyframe(*aKeyframes,
+                                                   aOffset,
+                                                   *aTimingFunction,
+                                                   keyframeIndex)) {
+        return &(*aKeyframes)[keyframeIndex];
+      }
+      break;
+    case KeyframeSearchDirection::Backwards:
+      if (nsAnimationManager::FindMatchingKeyframe(Reversed(*aKeyframes),
+                                                   aOffset,
+                                                   *aTimingFunction,
+                                                   keyframeIndex)) {
+        return &(*aKeyframes)[aKeyframes->Length() - 1 - keyframeIndex];
+      }
+      keyframeIndex = aKeyframes->Length() - 1;
+      break;
   }
 
-  Keyframe* keyframe = aKeyframes->InsertElementAt(0);
+  Keyframe* keyframe =
+    aKeyframes->InsertElementAt(
+      aInsertPosition == KeyframeInsertPosition::Prepend
+                         ? 0
+                         : keyframeIndex);
   keyframe->mOffset.emplace(aOffset);
   if (aTimingFunction->mType != nsTimingFunction::Type::Linear) {
     keyframe->mTimingFunction.emplace();
     keyframe->mTimingFunction->Init(*aTimingFunction);
   }
 
   return keyframe;
 }
 
+Keyframe*
+Gecko_GetOrCreateKeyframeAtStart(nsTArray<Keyframe>* aKeyframes,
+                                 float aOffset,
+                                 const nsTimingFunction* aTimingFunction)
+{
+  MOZ_ASSERT(aKeyframes->IsEmpty() ||
+             aKeyframes->ElementAt(0).mOffset.value() >= aOffset,
+             "The offset should be less than or equal to the first keyframe's "
+             "offset if there are exisiting keyframes");
+
+  return GetOrCreateKeyframe(aKeyframes,
+                             aOffset,
+                             aTimingFunction,
+                             KeyframeSearchDirection::Forwards,
+                             KeyframeInsertPosition::Prepend);
+}
+
+Keyframe*
+Gecko_GetOrCreateInitialKeyframe(nsTArray<Keyframe>* aKeyframes,
+                                 const nsTimingFunction* aTimingFunction)
+{
+  return GetOrCreateKeyframe(aKeyframes,
+                             0.,
+                             aTimingFunction,
+                             KeyframeSearchDirection::Forwards,
+                             KeyframeInsertPosition::LastForOffset);
+}
+
+Keyframe*
+Gecko_GetOrCreateFinalKeyframe(nsTArray<Keyframe>* aKeyframes,
+                               const nsTimingFunction* aTimingFunction)
+{
+  return GetOrCreateKeyframe(aKeyframes,
+                             1.,
+                             aTimingFunction,
+                             KeyframeSearchDirection::Backwards,
+                             KeyframeInsertPosition::LastForOffset);
+}
+
 void
 Gecko_ResetStyleCoord(nsStyleUnit* aUnit, nsStyleUnion* aValue)
 {
   nsStyleCoord::Reset(*aUnit, *aValue);
 }
 
 void
 Gecko_SetStyleCoordCalcValue(nsStyleUnit* aUnit, nsStyleUnion* aValue, nsStyleCoord::CalcValue aCalc)

--- a/layout/style/ServoBindings.h
+++ b/layout/style/ServoBindings.h
@@ -387,16 +387,33 @@ mozilla::Keyframe* Gecko_AnimationAppend
 //                        Keyframe is found, to set on the created Keyframe.
 //
 // @returns  The matching or created Keyframe.
 mozilla::Keyframe* Gecko_GetOrCreateKeyframeAtStart(
   RawGeckoKeyframeListBorrowedMut keyframes,
   float offset,
   const nsTimingFunction* timingFunction);
 
+// As with Gecko_GetOrCreateKeyframeAtStart except that this method will search
+// from the beginning of |keyframes| for a Keyframe with matching timing
+// function and an offset of 0.0.
+// Furthermore, if a matching Keyframe is not found, a new Keyframe will be
+// inserted after the *last* Keyframe in |keyframes| with offset 0.0.
+mozilla::Keyframe* Gecko_GetOrCreateInitialKeyframe(
+  RawGeckoKeyframeListBorrowedMut keyframes,
+  const nsTimingFunction* timingFunction);
+
+// As with Gecko_GetOrCreateKeyframeAtStart except that this method will search
+// from the *end* of |keyframes| for a Keyframe with matching timing function
+// and an offset of 1.0. If a matching Keyframe is not found, a new Keyframe
+// will be appended to the end of |keyframes|.
+mozilla::Keyframe* Gecko_GetOrCreateFinalKeyframe(
+  RawGeckoKeyframeListBorrowedMut keyframes,
+  const nsTimingFunction* timingFunction);
+
 // Clean up pointer-based coordinates
 void Gecko_ResetStyleCoord(nsStyleUnit* unit, nsStyleUnion* value);
 
 // Set an nsStyleCoord to a computed `calc()` value
 void Gecko_SetStyleCoordCalcValue(nsStyleUnit* unit, nsStyleUnion* value, nsStyleCoord::CalcValue calc);
 
 void Gecko_CopyShapeSourceFrom(mozilla::StyleShapeSource* dst, const mozilla::StyleShapeSource* src);