--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -12241,26 +12241,22 @@ CSSParserImpl::ParseImageLayersItem(
   RefPtr<nsCSSValue::Array> positionXArr = nsCSSValue::Array::Create(2);
   RefPtr<nsCSSValue::Array> positionYArr = nsCSSValue::Array::Create(2);
   aState.mPositionX->mValue.SetArrayValue(positionXArr, eCSSUnit_Array);
   aState.mPositionY->mValue.SetArrayValue(positionYArr, eCSSUnit_Array);
 
   if (eCSSProperty_mask == aTable[nsStyleImageLayers::shorthand]) {
     aState.mOrigin->mValue.SetIntValue(NS_STYLE_IMAGELAYER_ORIGIN_BORDER,
                                        eCSSUnit_Enumerated);
-
-    positionXArr->Item(1).SetPercentValue(0.5f);
-    positionYArr->Item(1).SetPercentValue(0.5f);
   } else {
     aState.mOrigin->mValue.SetIntValue(NS_STYLE_IMAGELAYER_ORIGIN_PADDING,
                                        eCSSUnit_Enumerated);
-
-    positionXArr->Item(1).SetPercentValue(0.0f);
-    positionYArr->Item(1).SetPercentValue(0.0f);
-  }
+  }
+  positionXArr->Item(1).SetPercentValue(0.0f);
+  positionYArr->Item(1).SetPercentValue(0.0f);
 
   aState.mSize->mXValue.SetAutoValue();
   aState.mSize->mYValue.SetAutoValue();
   aState.mComposite->mValue.SetIntValue(NS_STYLE_MASK_COMPOSITE_ADD,
                                         eCSSUnit_Enumerated);
   aState.mMode->mValue.SetIntValue(NS_STYLE_MASK_MODE_MATCH_SOURCE,
                                    eCSSUnit_Enumerated);
   bool haveColor = false,

--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -7303,19 +7303,17 @@ nsRuleNode::ComputeBackgroundData(void* 
                     &nsStyleImageLayers::Layer::mOrigin,
                     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]
   Position::Coord initialPositionCoord;
-  initialPositionCoord.mPercent =
-    nsStyleImageLayers::GetInitialPositionForLayerType(
-      nsStyleImageLayers::LayerType::Background);
+  initialPositionCoord.mPercent = 0.0f;
   initialPositionCoord.mLength = 0;
   initialPositionCoord.mHasPercent = true;
 
   SetImageLayerPositionCoordList(
                     aContext, *aRuleData->ValueForBackgroundPositionX(),
                     bg->mImage.mLayers,
                     parentBG->mImage.mLayers,
                     &Position::mXPosition,
@@ -9986,19 +9984,17 @@ nsRuleNode::ComputeSVGResetData(void* aS
                     &nsStyleImageLayers::Layer::mOrigin,
                     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]
   Position::Coord initialPositionCoord;
-  initialPositionCoord.mPercent =
-    nsStyleImageLayers::GetInitialPositionForLayerType(
-      nsStyleImageLayers::LayerType::Mask);
+  initialPositionCoord.mPercent = 0.0f;
   initialPositionCoord.mLength = 0;
   initialPositionCoord.mHasPercent = true;
 
   SetImageLayerPositionCoordList(
                     aContext, *aRuleData->ValueForMaskPositionX(),
                     svgReset->mMask.mLayers,
                     parentSVGReset->mMask.mLayers,
                     &Position::mXPosition,

--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -2620,21 +2620,20 @@ nsStyleImageLayers::HasLayerWithImage() 
   }
 
   return false;
 }
 
 bool
 nsStyleImageLayers::IsInitialPositionForLayerType(Position aPosition, LayerType aType)
 {
-  float intialValue = nsStyleImageLayers::GetInitialPositionForLayerType(aType);
-  if (aPosition.mXPosition.mPercent == intialValue &&
+  if (aPosition.mXPosition.mPercent == 0.0f &&
       aPosition.mXPosition.mLength == 0 &&
       aPosition.mXPosition.mHasPercent &&
-      aPosition.mYPosition.mPercent == intialValue &&
+      aPosition.mYPosition.mPercent == 0.0f &&
       aPosition.mYPosition.mLength == 0 &&
       aPosition.mYPosition.mHasPercent) {
     return true;
   }
 
   return false;
 }
 
@@ -2780,19 +2779,17 @@ nsStyleImageLayers::Layer::~Layer()
 {
 }
 
 void
 nsStyleImageLayers::Layer::Initialize(nsStyleImageLayers::LayerType aType)
 {
   mRepeat.SetInitialValues();
 
-  float initialPositionValue =
-    nsStyleImageLayers::GetInitialPositionForLayerType(aType);
-  mPosition.SetInitialPercentValues(initialPositionValue);
+  mPosition.SetInitialPercentValues(0.0f);
 
   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
@@ -665,20 +665,16 @@ struct nsStyleImageLayers {
   explicit nsStyleImageLayers(LayerType aType);
   nsStyleImageLayers(const nsStyleImageLayers &aSource);
   ~nsStyleImageLayers() {
     MOZ_COUNT_DTOR(nsStyleImageLayers);
   }
 
   static bool IsInitialPositionForLayerType(mozilla::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;