--- a/layout/style/nsDOMCSSDeclaration.cpp
+++ b/layout/style/nsDOMCSSDeclaration.cpp
@@ -75,16 +75,18 @@ nsDOMCSSDeclaration::GetCustomPropertyVa
 }
 
 NS_IMETHODIMP
 nsDOMCSSDeclaration::SetPropertyValue(const nsCSSProperty aPropID,
                                       const nsAString& aValue)
 {
   switch (aPropID) {
     case eCSSProperty_background_position:
+    case eCSSProperty_background_position_x:
+    case eCSSProperty_background_position_y:
     case eCSSProperty_transform:
     case eCSSProperty_top:
     case eCSSProperty_left:
     case eCSSProperty_bottom:
     case eCSSProperty_right:
     case eCSSProperty_margin:
     case eCSSProperty_margin_top:
     case eCSSProperty_margin_left: