getComputedStyle should handle over-constrained properties.

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
  * modify it under the terms of the GNU Lesser General Public
@@ skipping 229 matching lines @@
             default:
                 ASSERT_NOT_REACHED();
             }
             return zoomAdjustedPixelValue(position, style);
         }
     }
 
     if (offset.isAuto())
         return cssValuePool().createIdentifierValue(CSSValueAuto);
 
+    if (layoutObject) {
+        if (!opposite.isAuto() && layoutObject->isInFlowPositioned()) {
+            LayoutBlock* container = layoutObject->containingBlock();
+            bool ltr = container->style()->isLeftToRightDirection();
+            if ((propertyID == CSSPropertyLeft && !ltr) || (propertyID == CSSPropertyRight && ltr) || (propertyID == CSSPropertyBottom))
+                return zoomAdjustedPixelValue(-opposite.pixels(), style);
+        }
+    }
     return zoomAdjustedPixelValueForLength(offset, style);
 }
 
 static CSSBorderImageSliceValue* valueForNinePieceImageSlice(const NinePieceImage& image)
 {
     // Create the slices.
     CSSPrimitiveValue* top = nullptr;
     CSSPrimitiveValue* right = nullptr;
     CSSPrimitiveValue* bottom = nullptr;
     CSSPrimitiveValue* left = nullptr;
@@ skipping 2740 matching lines @@
     case CSSPropertyAll:
         return nullptr;
     default:
         break;
     }
     ASSERT_NOT_REACHED();
     return nullptr;
 }
 
 } // namespace blink