diff --git a/layout/style/Declaration.cpp b/layout/style/Declaration.cpp
index f4197439f8..abda70f277 100644
--- a/layout/style/Declaration.cpp
+++ b/layout/style/Declaration.cpp
@@ -802,18 +802,63 @@ Declaration::GetPropertyValueInternal(
       MOZ_ASSERT(StringEndsWith(nsCSSProps::GetStringValue(subprops[2]),
                                 NS_LITERAL_CSTRING("-color")),
                  "third subprop must be the color property");
+
+      const nsCSSValue *widthValue = data->ValueFor(subprops[0]);
+      const nsCSSValue *styleValue = data->ValueFor(subprops[1]);
       const nsCSSValue *colorValue = data->ValueFor(subprops[2]);
+
+      bool isNoneStyle = 
+        styleValue->GetUnit() == eCSSUnit_Enumerated &&
+        styleValue->GetIntValue() == NS_STYLE_BORDER_STYLE_NONE;
+
+      bool isMediumWidth = 
+        widthValue->GetUnit() == eCSSUnit_Enumerated &&
+        widthValue->GetIntValue() == NS_STYLE_BORDER_WIDTH_MEDIUM;
+
+      bool isCurrentColor = colorValue->GetUnit() == eCSSUnit_EnumColor &&
+                            colorValue->GetIntValue() == NS_COLOR_CURRENTCOLOR;
+
+      if (isNoneStyle && isCurrentColor) {
+        // Case (1) above: some subproperties not specified.
+        return;
+      }
+
       bool isCurrentColor =
         colorValue->GetUnit() == eCSSUnit_EnumColor &&
         colorValue->GetIntValue() == NS_COLOR_CURRENTCOLOR;
-      if (!AppendValueToString(subprops[0], aValue, aSerialization) ||
-          !(aValue.Append(char16_t(' ')),
-            AppendValueToString(subprops[1], aValue, aSerialization)) ||
-          // Don't output a third value when it's currentcolor.
-          !(isCurrentColor ||
-            (aValue.Append(char16_t(' ')),
-             AppendValueToString(subprops[2], aValue, aSerialization)))) {
-        aValue.Truncate();
+
+      // special case: if we have just color and all other values are default,
+      // output: color
+      if (!isCurrentColor && isNoneStyle && isMediumWidth) {
+        if (!AppendValueToString(subprops[2], aValue, aSerialization)) {
+          aValue.Truncate();
+        }
+        break;
+      }
+
+      // normal case: output width unless medium and only have color
+      if (!(isMediumWidth && isNoneStyle && !isCurrentColor)) {
+        if (!AppendValueToString(subprops[0], aValue, aSerialization)) {
+          aValue.Truncate();
+          break;
+        }
+      }
+
+      // if style is not none, append style
+      if (!isNoneStyle) {
+        aValue.Append(char16_t(' '));
+        if (!AppendValueToString(subprops[1], aValue, aSerialization)) {
+          aValue.Truncate();
+          break;
+        }
+      }
+
+      if (!isCurrentColor) {
+        aValue.Append(char16_t(' '));
+        if (!AppendValueToString(subprops[2], aValue, aSerialization)) {
+          aValue.Truncate();
+          break;
+        }
       }
       break;
     }
@@ -1030,12 +1075,21 @@ Declaration::GetPropertyValueInternal(
                           aSerialization);
       break;
     case eCSSProperty_overflow: {
-      const nsCSSValue &xValue =
-        *data->ValueFor(eCSSProperty_overflow_x);
-      const nsCSSValue &yValue =
-        *data->ValueFor(eCSSProperty_overflow_y);
-      if (xValue == yValue)
-        xValue.AppendToString(eCSSProperty_overflow_x, aValue, aSerialization);
+      const nsCSSValue* xValue = data->ValueFor(eCSSProperty_overflow_x);
+      const nsCSSValue* yValue = data->ValueFor(eCSSProperty_overflow_y);
+      if (!xValue || !yValue ||
+          xValue->GetUnit() != eCSSUnit_Enumerated ||
+          yValue->GetUnit() != eCSSUnit_Enumerated) {
+        aValue.Truncate(); // don't serialize shorthand if not both present/enumerated
+        break;
+      }
+      if (*xValue == *yValue) {
+        xValue->AppendToString(eCSSProperty_overflow_x, aValue, aSerialization);
+      } else {
+        xValue->AppendToString(eCSSProperty_overflow_x, aValue, aSerialization);
+        aValue.Append(char16_t(' '));
+        yValue->AppendToString(eCSSProperty_overflow_y, aValue, aSerialization);
+      }
       break;
     }
     case eCSSProperty_text_decoration: {