Let StyleResolver figure out differences between UA style and resolved style.

Source/core/css/resolver/StyleResolver.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 2004-2005 Allan Sandfeld Jensen (kde@carewolf.com)
  * Copyright (C) 2006, 2007 Nicholas Shanks (webkit@nickshanks.com)
  * Copyright (C) 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alexey Proskuryakov <ap@webkit.org>
  * Copyright (C) 2007, 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (c) 2011, Code Aurora Forum. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
@@ skipping 508 matching lines @@
     // so that the common case doesn't need to create a new LayoutStyle in
     // Document::inheritHtmlAndBodyElementStyles.
     documentStyle->setDisplay(BLOCK);
     documentStyle->setScrollBlocksOn(WebScrollBlocksOnStartTouch | WebScrollBlocksOnWheelEvent);
 
     document.setupFontBuilder(*documentStyle);
 
     return documentStyle.release();
 }
 
+static AuthorStyleInfo authorStyleInfo(StyleResolverState& state)
+{
+    const CachedUAStyle* cachedUAStyle = state.cachedUAStyle();
+
+    if (!cachedUAStyle)
+        return AuthorStyleInfo();
+
+    // Exclude background-repeat from comparison by resetting it.
+    FillLayer backgroundCopy = cachedUAStyle->backgroundLayers;
+    FillLayer backgroundLayersCopy = state.style()->backgroundLayers();
+    backgroundCopy.setRepeatX(NoRepeatFill);
+    backgroundCopy.setRepeatY(NoRepeatFill);
+    backgroundLayersCopy.setRepeatX(NoRepeatFill);
+    backgroundLayersCopy.setRepeatY(NoRepeatFill);
+
+    bool backgroundChanged = backgroundLayersCopy != backgroundCopy
+        || state.style()->visitedDependentColor(CSSPropertyBackgroundColor) != cachedUAStyle->backgroundColor;
+    bool borderChanged = state.style()->border() != cachedUAStyle->border;
+
+    return AuthorStyleInfo(backgroundChanged, borderChanged);
+}
+
 void StyleResolver::adjustLayoutStyle(StyleResolverState& state, Element* element)
 {
     StyleAdjuster adjuster(document().inQuirksMode());
-    adjuster.adjustLayoutStyle(state.mutableStyleRef(), *state.parentStyle(), element, state.cachedUAStyle());
+    adjuster.adjustLayoutStyle(state.mutableStyleRef(), *state.parentStyle(), element, authorStyleInfo(state));
 }
 
 // Start loading resources referenced by this style.
 void StyleResolver::loadPendingResources(StyleResolverState& state)
 {
     m_styleResourceLoader.loadPendingResources(state.style(), state.elementStyleResources());
     document().styleEngine()->fontSelector()->fontLoader()->loadPendingFonts();
 }
 
 PassRefPtr<LayoutStyle> StyleResolver::styleForElement(Element* element, LayoutStyle* defaultParent, StyleSharingBehavior sharingBehavior,
@@ skipping 982 matching lines @@
     visitor->trace(m_uncommonAttributeRuleSet);
     visitor->trace(m_watchedSelectorsRules);
     visitor->trace(m_treeBoundaryCrossingRules);
     visitor->trace(m_styleSharingLists);
     visitor->trace(m_pendingStyleSheets);
     visitor->trace(m_document);
 #endif
 }
 
 } // namespace blink