--- a/layout/style/nsCSSParser.cpp
+++ b/layout/style/nsCSSParser.cpp
@@ -12772,28 +12772,28 @@ AdjustEdgeOffsetPairForBasicShape(nsCSSV
                                   nsCSSValue& aOffset,
                                   uint8_t aDefaultEdge)
 {
   // 0 length offsets are 0%
   if (aOffset.IsLengthUnit() && aOffset.GetFloatValue() == 0.0) {
     aOffset.SetPercentValue(0);
   }
 
-  // Default edge is top/left in the 4-value case
-  // In case of 1 or 0 values, the default is center,
-  // but ParsePositionValue already handles this case
-  if (eCSSUnit_Null == aEdge.GetUnit()) {
-    aEdge.SetIntValue(aDefaultEdge, eCSSUnit_Enumerated);
-  }
   // Default offset is 0%
   if (eCSSUnit_Null == aOffset.GetUnit()) {
     aOffset.SetPercentValue(0.0);
   }
-  if (eCSSUnit_Enumerated == aEdge.GetUnit() &&
-      eCSSUnit_Percent == aOffset.GetUnit()) {
+
+  if (eCSSUnit_Percent == aOffset.GetUnit()) {
+    // Default edge is top/left in the 4-value case
+    // In case of 1 or 0 values, the default is center,
+    // but ParsePositionValue already handles this case
+    if (eCSSUnit_Null == aEdge.GetUnit()) {
+      aEdge.SetIntValue(aDefaultEdge, eCSSUnit_Enumerated);
+    }
     switch (aEdge.GetIntValue()) {
       case NS_STYLE_IMAGELAYER_POSITION_CENTER:
         aEdge.SetIntValue(aDefaultEdge, eCSSUnit_Enumerated);
         MOZ_ASSERT(aOffset.GetPercentValue() == 0.0,
                    "center cannot be used with an offset");
         aOffset.SetPercentValue(0.5);
         break;
       case NS_STYLE_IMAGELAYER_POSITION_BOTTOM:

--- a/layout/style/nsCSSValue.cpp
+++ b/layout/style/nsCSSValue.cpp
@@ -988,32 +988,41 @@ void
 nsCSSValue::AppendBasicShapePositionToString(nsAString& aResult,
                                              Serialization aSerialization) const
 {
   const nsCSSValue::Array* array = GetArrayValue();
   // We always parse these into an array of four elements
   MOZ_ASSERT(array->Count() == 4,
              "basic-shape position value doesn't have enough elements");
 
-  const nsCSSValue &xEdge   = array->Item(0);
+  nsCSSValue xEdge   = array->Item(0);
   const nsCSSValue &xOffset = array->Item(1);
-  const nsCSSValue &yEdge   = array->Item(2);
+  nsCSSValue yEdge   = array->Item(2);
   const nsCSSValue &yOffset = array->Item(3);
 
-  MOZ_ASSERT(xEdge.GetUnit() == eCSSUnit_Enumerated &&
-             yEdge.GetUnit() == eCSSUnit_Enumerated &&
-             xOffset.IsLengthPercentCalcUnit() &&
-             yOffset.IsLengthPercentCalcUnit() &&
-             xEdge.GetIntValue() != NS_STYLE_IMAGELAYER_POSITION_CENTER &&
-             yEdge.GetIntValue() != NS_STYLE_IMAGELAYER_POSITION_CENTER,
-             "Ensure invariants from ParsePositionValueBasicShape "
-             "haven't been modified");
+  // Ensure invariants from ParsePositionValueBasicShape aven't been modified.
+  MOZ_ASSERT_IF(xOffset.GetUnit() == eCSSUnit_Percent,
+                xEdge.GetUnit() == eCSSUnit_Enumerated &&
+                xEdge.GetIntValue() != NS_STYLE_IMAGELAYER_POSITION_CENTER);
+  MOZ_ASSERT_IF(yOffset.GetUnit() == eCSSUnit_Percent,
+                yEdge.GetUnit() == eCSSUnit_Enumerated &&
+                yEdge.GetIntValue() != NS_STYLE_IMAGELAYER_POSITION_CENTER);
+
+  // Give default value to these edges.
+  if (xEdge.GetUnit() == eCSSUnit_Null) {
+    xEdge.SetIntValue(NS_STYLE_IMAGELAYER_POSITION_LEFT, eCSSUnit_Enumerated);
+  }
+  if (yEdge.GetUnit() == eCSSUnit_Null) {
+    yEdge.SetIntValue(NS_STYLE_IMAGELAYER_POSITION_TOP, eCSSUnit_Enumerated);
+  }
+
   if (xEdge.GetIntValue() == NS_STYLE_IMAGELAYER_POSITION_LEFT &&
       yEdge.GetIntValue() == NS_STYLE_IMAGELAYER_POSITION_TOP) {
-    // We can omit these defaults
+    // We should omit these defaults since 2-value form is preferred in
+    // basic-shape serialization.
     xOffset.AppendToString(eCSSProperty_UNKNOWN, aResult, aSerialization);
     aResult.Append(' ');
     yOffset.AppendToString(eCSSProperty_UNKNOWN, aResult, aSerialization);
   } else {
     // We only serialize to the two or four valued form
     xEdge.AppendToString(eCSSProperty_object_position, aResult, aSerialization);
     aResult.Append(' ');
     xOffset.AppendToString(eCSSProperty_UNKNOWN, aResult, aSerialization);