diff --git a/layout/style/nsComputedDOMStyle.cpp b/layout/style/nsComputedDOMStyle.cpp
index 7a84b1d34e64..47f70ba385a7 100644
--- a/layout/style/nsComputedDOMStyle.cpp
+++ b/layout/style/nsComputedDOMStyle.cpp
@@ -1594,20 +1594,18 @@ nsComputedDOMStyle::DoGetMozBackgroundSize()
           NS_ABORT_IF_FALSE(size.mWidthType ==
                               nsStyleBackground::Size::eLengthPercentage,
                             "bad mWidthType");
-          if (size.mWidth.mLength == 0 &&
+          if (!size.mWidth.mHasPercent &&
               // negative values must have come from calc()
-              size.mWidth.mPercent > 0.0f) {
-            valX->SetPercent(size.mWidth.mPercent);
-          } else if (size.mWidth.mPercent == 0.0f &&
-                     // negative values must have come from calc()
-                     size.mWidth.mLength > 0) {
+              size.mWidth.mLength >= 0) {
+            NS_ABORT_IF_FALSE(size.mWidth.mPercent == 0.0f,
+                              "Shouldn't have mPercent");
             valX->SetAppUnits(size.mWidth.mLength);
+          } else if (size.mWidth.mLength == 0 &&
+                     // negative values must have come from calc()
+                     size.mWidth.mPercent >= 0.0f) {
+            valX->SetPercent(size.mWidth.mPercent);
           } else {
-            nsStyleCoord::Calc calc;
-            calc.mPercent = size.mWidth.mPercent;
-            calc.mLength  = size.mWidth.mLength;
-            calc.mHasPercent = PR_TRUE;
-            SetValueToCalc(&calc, valX);
+            SetValueToCalc(&size.mWidth, valX);
           }
         }
 
@@ -1617,20 +1615,18 @@ nsComputedDOMStyle::DoGetMozBackgroundSize()
           NS_ABORT_IF_FALSE(size.mHeightType ==
                               nsStyleBackground::Size::eLengthPercentage,
                             "bad mHeightType");
-          if (size.mHeight.mLength == 0 &&
+          if (!size.mHeight.mHasPercent &&
               // negative values must have come from calc()
-              size.mHeight.mPercent > 0.0f) {
-            valY->SetPercent(size.mHeight.mPercent);
-          } else if (size.mHeight.mPercent == 0.0f &&
-                     // negative values must have come from calc()
-                     size.mHeight.mLength > 0) {
+              size.mHeight.mLength >= 0) {
+            NS_ABORT_IF_FALSE(size.mHeight.mPercent == 0.0f,
+                              "Shouldn't have mPercent");
             valY->SetAppUnits(size.mHeight.mLength);
+          } else if (size.mHeight.mLength == 0 &&
+                     // negative values must have come from calc()
+                     size.mHeight.mPercent >= 0.0f) {
+            valY->SetPercent(size.mHeight.mPercent);
           } else {
-            nsStyleCoord::Calc calc;
-            calc.mPercent = size.mHeight.mPercent;
-            calc.mLength  = size.mHeight.mLength;
-            calc.mHasPercent = PR_TRUE;
-            SetValueToCalc(&calc, valY);
+            SetValueToCalc(&size.mHeight, valY);
           }
         }
         break;
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index e55a016053c5..8b06de84fb18 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -4746,6 +4746,7 @@ struct BackgroundItemComputer<nsCSSValuePairList, nsStyleBackground::Size>
       else if (eCSSUnit_Percent == specified.GetUnit()) {
         (size.*(axis->result)).mLength = 0;
         (size.*(axis->result)).mPercent = specified.GetPercentValue();
+        (size.*(axis->result)).mHasPercent = PR_TRUE;
         size.*(axis->type) = nsStyleBackground::Size::eLengthPercentage;
       }
       else if (specified.IsLengthUnit()) {
@@ -4753,6 +4754,7 @@ struct BackgroundItemComputer<nsCSSValuePairList, nsStyleBackground::Size>
           CalcLength(specified, aStyleContext, aStyleContext->PresContext(),
                      aCanStoreInRuleTree);
         (size.*(axis->result)).mPercent = 0.0f;
+        (size.*(axis->result)).mHasPercent = PR_FALSE;
         size.*(axis->type) = nsStyleBackground::Size::eLengthPercentage;
       } else {
         NS_ABORT_IF_FALSE(specified.IsCalcUnit(), "unexpected unit");
@@ -4762,6 +4764,7 @@ struct BackgroundItemComputer<nsCSSValuePairList, nsStyleBackground::Size>
         nsRuleNode::ComputedCalc vals = ComputeCalc(specified, ops);
         (size.*(axis->result)).mLength = vals.mLength;
         (size.*(axis->result)).mPercent = vals.mPercent;
+        (size.*(axis->result)).mHasPercent = ops.mHasPercent;
         size.*(axis->type) = nsStyleBackground::Size::eLengthPercentage;
       }
     }
diff --git a/layout/style/nsStyleStruct.h b/layout/style/nsStyleStruct.h
index 618ef0b85ee9..d83eb9894434 100644
--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -378,17 +378,7 @@ struct nsStyleBackground {
   struct Size;
   friend struct Size;
   struct Size {
-    struct Dimension {
-      // A 'background-size' can be a linear combination of length
-      // and percent (thanks to calc(), which can combine them).
-      nscoord mLength;
-      float   mPercent;
-
-      bool operator==(const Dimension& aOther) const
-        { return mLength == aOther.mLength && mPercent == aOther.mPercent; }
-      bool operator!=(const Dimension& aOther) const
-        { return !(*this == aOther); }
-    };
+    typedef nsStyleCoord::Calc Dimension;
     Dimension mWidth, mHeight;
 
     // Except for eLengthPercentage, Dimension types which might change
diff --git a/layout/style/test/test_computed_style.html b/layout/style/test/test_computed_style.html
index 4fd97d028a71..55b05dcd95ec 100644
--- a/layout/style/test/test_computed_style.html
+++ b/layout/style/test/test_computed_style.html
@@ -149,6 +149,40 @@ var noframe_container = document.getElementById("content");
   p.parentNode.removeChild(p);
 })();
 
+(function test_bug_647885_2() {
+  // Test that various background-size styles round-trip correctly
+  var backgroundSizes = [
+    [ "0 0", "0px 0px", "unitless 0" ],
+    [ "0px 0px", "0px 0px", "0 with units" ],
+    [ "0% 0%", "0% 0%", "0%" ],
+    [ "-moz-calc(0px) 0", "0px 0px", "0 calc with units horizontal" ],
+    [ "0 -moz-calc(0px)", "0px 0px", "0 calc with units vertical" ],
+    [ "-moz-calc(3px - 3px) 0", "0px 0px", "computed 0 calc with units horizontal" ],
+    [ "0 -moz-calc(3px - 3px)", "0px 0px", "computed 0 calc with units vertical" ],
+    [ "-moz-calc(0%) 0", "0% 0px", "0% calc horizontal"],
+    [ "0 -moz-calc(0%)", "0px 0%", "0% calc vertical"],
+    [ "-moz-calc(3px + 2% - 2%) 0", "-moz-calc(3px + 0%) 0px",
+                      "computed 0% calc horizontal"],
+    [ "0 -moz-calc(3px + 2% - 2%)", "0px -moz-calc(3px + 0%)",
+                      "computed 0% calc vertical"],
+    [ "-moz-calc(3px - 5px) -moz-calc(6px - 9px)",
+      "-moz-calc(-2px) -moz-calc(-3px)", "negative pixel width" ],
+    [ "", "auto auto", "initial value" ],
+  ];
+
+  var p = document.createElement("p");
+  var cs = getComputedStyle(p, "");
+  frame_container.appendChild(p);
+
+  for (var i = 0; i < backgroundSizes.length; ++i) {
+    var test = backgroundSizes[i];
+    p.style.backgroundSize = test[0];
+    is(cs.backgroundSize, test[1], "computed value of " + test[2] + " background-size");
+  }
+
+  p.parentNode.removeChild(p);
+})();
+
 </script>
 </pre>
 </body>