Hovered/active/focused will never share style with siblings/cousins.

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 800 matching lines @@
         return 0;
 
     // If the element has inline style it is probably unique.
     if (state.styledElement()->inlineStyle())
         return 0;
     if (state.styledElement()->isSVGElement() && toSVGElement(state.styledElement())->animatedSMILStyleProperties())
         return 0;
     // Ids stop style sharing if they show up in the stylesheets.
     if (state.styledElement()->hasID() && m_features.idsInRules.contains(state.styledElement()->idForStyleResolution().impl()))
         return 0;
+    // Active and hovered elements always make a chain towards the document node
+    // and no siblings or cousins will have the same state.
+    if (state.styledElement()->hovered())
+        return 0;
+    if (state.styledElement()->active())
+        return 0;
+    // There is always only one focused element.
+    if (state.styledElement()->focused())
+        return 0;
     if (parentElementPreventsSharing(state.element()->parentElement()))
         return 0;
     if (state.styledElement()->hasScopedHTMLStyleChild())
         return 0;
     if (state.element() == state.document()->cssTarget())
         return 0;
     if (elementHasDirectionAuto(state.element()))
         return 0;
     if (state.element()->hasActiveAnimations())
         return 0;
@@ skipping 2648 matching lines @@
     info.addMember(m_state, "state");
 
     // FIXME: move this to a place where it would be called only once?
     info.addMember(CSSDefaultStyleSheets::defaultStyle, "defaultStyle");
     info.addMember(CSSDefaultStyleSheets::defaultQuirksStyle, "defaultQuirksStyle");
     info.addMember(CSSDefaultStyleSheets::defaultPrintStyle, "defaultPrintStyle");
     info.addMember(CSSDefaultStyleSheets::defaultViewSourceStyle, "defaultViewSourceStyle");
 }
 
 } // namespace WebCore