--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -6143,17 +6143,18 @@ nsComputedDOMStyle::DoGetMask()
   // Mask is now a shorthand, but it used to be a longhand, so that we
   // need to support computed style for the cases where it used to be
   // a longhand.
   if (svg->mMask.mImageCount > 1 ||
       firstLayer.mClip != NS_STYLE_IMAGELAYER_CLIP_BORDER ||
       firstLayer.mOrigin != NS_STYLE_IMAGELAYER_ORIGIN_BORDER ||
       firstLayer.mComposite != NS_STYLE_MASK_COMPOSITE_ADD ||
       firstLayer.mMaskMode != NS_STYLE_MASK_MODE_MATCH_SOURCE ||
-      !firstLayer.mPosition.IsInitialValue(nsStyleImageLayers::LayerType::Mask) ||
+      !nsStyleImageLayers::IsInitialPositionForLayerType(
+        firstLayer.mPosition, nsStyleImageLayers::LayerType::Mask) ||
       !firstLayer.mRepeat.IsInitialValue(nsStyleImageLayers::LayerType::Mask) ||
       !firstLayer.mSize.IsInitialValue() ||
       !(firstLayer.mImage.GetType() == eStyleImageType_Null ||
         firstLayer.mImage.GetType() == eStyleImageType_Image)) {
     return nullptr;
   }
 
   RefPtr<nsROCSSPrimitiveValue> val = new nsROCSSPrimitiveValue;

--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -7268,17 +7268,17 @@ nsRuleNode::ComputeBackgroundData(void* 
                     uint8_t(NS_STYLE_IMAGELAYER_ORIGIN_PADDING),
                     parentBG->mImage.mOriginCount,
                     bg->mImage.mOriginCount, maxItemCount, rebuild,
                     conditions);
 
   // background-position-x/y: enum, length, percent (flags), inherit [list]
   nsStyleImageLayers::Position::PositionCoord initialPositionCoord;
   initialPositionCoord.mPercent =
-    nsStyleImageLayers::Position::GetInitialValue(
+    nsStyleImageLayers::GetInitialPositionForLayerType(
       nsStyleImageLayers::LayerType::Background);
   initialPositionCoord.mLength = 0;
   initialPositionCoord.mHasPercent = true;
 
   SetImageLayerPositionCoordList(
                     aContext, *aRuleData->ValueForBackgroundPositionX(),
                     bg->mImage.mLayers,
                     parentBG->mImage.mLayers,
@@ -10032,17 +10032,17 @@ nsRuleNode::ComputeSVGResetData(void* aS
                     uint8_t(NS_STYLE_IMAGELAYER_ORIGIN_BORDER),
                     parentSVGReset->mMask.mOriginCount,
                     svgReset->mMask.mOriginCount, maxItemCount, rebuild,
                     conditions);
 
   // mask-position-x/y: enum, length, percent (flags), inherit [list]
   nsStyleImageLayers::Position::PositionCoord initialPositionCoord;
   initialPositionCoord.mPercent =
-    nsStyleImageLayers::Position::GetInitialValue(
+    nsStyleImageLayers::GetInitialPositionForLayerType(
       nsStyleImageLayers::LayerType::Mask);
   initialPositionCoord.mLength = 0;
   initialPositionCoord.mHasPercent = true;
 
   SetImageLayerPositionCoordList(
                     aContext, *aRuleData->ValueForMaskPositionX(),
                     svgReset->mMask.mLayers,
                     parentSVGReset->mMask.mLayers,

--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -2485,22 +2485,25 @@ nsStyleImageLayers::HasLayerWithImage() 
       return true;
     }
   }
 
   return false;
 }
 
 bool
-nsStyleImageLayers::Position::IsInitialValue(LayerType aType) const
+nsStyleImageLayers::IsInitialPositionForLayerType(Position aPosition, LayerType aType)
 {
-  float intialValue = nsStyleImageLayers::Position::GetInitialValue(aType);
-  if (mXPosition.mPercent == intialValue && mXPosition.mLength == 0 &&
-      mXPosition.mHasPercent && mYPosition.mPercent == intialValue &&
-      mYPosition.mLength == 0 && mYPosition.mHasPercent) {
+  float intialValue = nsStyleImageLayers::GetInitialPositionForLayerType(aType);
+  if (aPosition.mXPosition.mPercent == intialValue &&
+      aPosition.mXPosition.mLength == 0 &&
+      aPosition.mXPosition.mHasPercent &&
+      aPosition.mYPosition.mPercent == intialValue &&
+      aPosition.mYPosition.mLength == 0 &&
+      aPosition.mYPosition.mHasPercent) {
     return true;
   }
 
   return false;
 }
 
 void
 nsStyleImageLayers::Position::SetInitialPercentValues(float aPercentVal)
@@ -2670,17 +2673,17 @@ nsStyleImageLayers::Layer::~Layer()
 }
 
 void
 nsStyleImageLayers::Layer::Initialize(nsStyleImageLayers::LayerType aType)
 {
   mRepeat.SetInitialValues(aType);
 
   float initialPositionValue =
-    nsStyleImageLayers::Position::GetInitialValue(aType);
+    nsStyleImageLayers::GetInitialPositionForLayerType(aType);
   mPosition.SetInitialPercentValues(initialPositionValue);
 
   if (aType == LayerType::Background) {
     mOrigin = NS_STYLE_IMAGELAYER_ORIGIN_PADDING;
   } else {
     MOZ_ASSERT(aType == LayerType::Mask, "unsupported layer type.");
     mOrigin = NS_STYLE_IMAGELAYER_ORIGIN_BORDER;
   }

--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -565,22 +565,16 @@ struct nsStyleImageLayers {
   friend struct Position;
   struct Position {
     typedef nsStyleCoord::CalcValue PositionCoord;
     PositionCoord mXPosition, mYPosition;
 
     // Initialize nothing
     Position() {}
 
-    bool IsInitialValue(LayerType aType) const;
-
-    static float GetInitialValue(LayerType aType) {
-      return (aType == LayerType::Background) ? 0.0f : 0.5f;
-    }
-
     // Sets both mXPosition and mYPosition to the given percent value for the
     // initial property-value (e.g. 0.0f for "0% 0%", or 0.5f for "50% 50%")
     void SetInitialPercentValues(float aPercentVal);
 
     // Sets both mXPosition and mYPosition to 0 (app units) for the
     // initial property-value as a length with no percentage component.
     void SetInitialZeroValues();
 
@@ -594,16 +588,22 @@ struct nsStyleImageLayers {
       return mXPosition == aOther.mXPosition &&
              mYPosition == aOther.mYPosition;
     }
     bool operator!=(const Position& aOther) const {
       return !(*this == aOther);
     }
   };
 
+  static bool IsInitialPositionForLayerType(Position aPosition, LayerType aType);
+
+  static float GetInitialPositionForLayerType(LayerType aType) {
+    return (aType == LayerType::Background) ? 0.0f : 0.5f;
+  }
+
   struct Size;
   friend struct Size;
   struct Size {
     struct Dimension : public nsStyleCoord::CalcValue {
       nscoord ResolveLengthPercentage(nscoord aAvailable) const {
         double d = double(mPercent) * double(aAvailable) + double(mLength);
         if (d < 0.0) {
           return 0;