Return '' for overflow if overflow-x and overflow-y are different.

third_party/WebKit/Source/core/css/ComputedStyleCSSValueMapping.cpp

 /*
  * Copyright (C) 2004 Zack Rusin <zack@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc.
  * All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007 Nicholas Shanks <webkit@nickshanks.com>
  * Copyright (C) 2011 Sencha, Inc. All rights reserved.
  * Copyright (C) 2015 Google Inc. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
@@ skipping 2636 matching lines @@
                  : currentColorOrValidColor(style, style.outlineColor());
     case CSSPropertyOutlineOffset:
       return zoomAdjustedPixelValue(style.outlineOffset(), style);
     case CSSPropertyOutlineStyle:
       if (style.outlineStyleIsAuto())
         return CSSIdentifierValue::create(CSSValueAuto);
       return CSSIdentifierValue::create(style.outlineStyle());
     case CSSPropertyOutlineWidth:
       return zoomAdjustedPixelValue(style.outlineWidth(), style);
     case CSSPropertyOverflow:
-      return CSSIdentifierValue::create(
-          max(style.overflowX(), style.overflowY()));
+      if (style.overflowX() == style.overflowY())
+        return CSSIdentifierValue::create(style.overflowX());
+      return nullptr;
     case CSSPropertyOverflowAnchor:
       return CSSIdentifierValue::create(style.overflowAnchor());
     case CSSPropertyOverflowWrap:
       return CSSIdentifierValue::create(style.overflowWrap());
     case CSSPropertyOverflowX:
       return CSSIdentifierValue::create(style.overflowX());
     case CSSPropertyOverflowY:
       return CSSIdentifierValue::create(style.overflowY());
     case CSSPropertyPaddingTop: {
       Length paddingTop = style.paddingTop();
@@ skipping 994 matching lines @@
     case CSSPropertyAll:
       return nullptr;
     default:
       break;
   }
   ASSERT_NOT_REACHED();
   return nullptr;
 }
 
 }  // namespace blink