Consider pseudo classes as matching for shared style rejection.

third_party/WebKit/Source/core/css/SelectorChecker.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 580 matching lines @@
     subContext.isSubSelector = true;
     ASSERT(selector.selectorList());
     for (subContext.selector = selector.selectorList()->first(); subContext.selector; subContext.selector = subContext.selector->tagHistory()) {
         // :not cannot nest. I don't really know why this is a
         // restriction in CSS3, but it is, so let's honor it.
         // the parser enforces that this never occurs
         ASSERT(subContext.selector->getPseudoType() != CSSSelector::PseudoNot);
         // We select between :visited and :link when applying. We don't know which one applied (or not) yet.
         if (subContext.selector->getPseudoType() == CSSSelector::PseudoVisited || (subContext.selector->getPseudoType() == CSSSelector::PseudoLink && subContext.visitedMatchType == VisitedMatchEnabled))
             return true;
-        // context.scope is not available if m_mode == SharingRules.
-        // We cannot determine whether :host or :scope matches a given element or not.
-        if (m_mode == SharingRules && (subContext.selector->isHostPseudoClass() || subContext.selector->getPseudoType() == CSSSelector::PseudoScope))
-            return true;
+        if (m_mode == SharingRules) {
+            // context.scope is not available if m_mode == SharingRules.
+            // We cannot determine whether :host or :scope matches a given element or not.
+            if (subContext.selector->isHostPseudoClass() || subContext.selector->getPseudoType() == CSSSelector::PseudoScope)
+                return true;
+            // :hover, :active, :focus, :-webkit-drag relies on setting flags on
+            // ComputedStyle even if the whole selector may not match. That
+            // means we cannot share style between elements which may fail
+            // matching the same selector for different reasons. An example is
+            // [attr]:hover which both fail for :hover, but an element without
+            // attr won't reach the :hover selector, hence not setting the bit.
+            if (subContext.selector->isUserActionPseudoClass())
+                return true;
+        }
         if (!checkOne(subContext, result))
             return true;
     }
     return false;
 }
 
 bool SelectorChecker::checkPseudoClass(const SelectorCheckingContext& context, MatchResult& result) const
 {
     Element& element = *context.element;
     const CSSSelector& selector = *context.selector;
@@ skipping 143 matching lines @@
         }
         break;
     case CSSSelector::PseudoAutofill:
         return element.isFormControlElement() && toHTMLFormControlElement(element).isAutofilled();
     case CSSSelector::PseudoAnyLink:
     case CSSSelector::PseudoLink:
         return element.isLink();
     case CSSSelector::PseudoVisited:
         return element.isLink() && context.visitedMatchType == VisitedMatchEnabled;
     case CSSSelector::PseudoDrag:
+        if (m_mode == SharingRules)
+            return true;
         if (m_mode == ResolvingStyle) {
             if (context.inRightmostCompound) {
                 m_elementStyle->setAffectedByDrag();
             } else {
                 m_elementStyle->setUnique();
                 element.setChildrenOrSiblingsAffectedByDrag();
             }
         }
         return element.isDragged();
     case CSSSelector::PseudoFocus:
+        if (m_mode == SharingRules)
+            return true;
         if (m_mode == ResolvingStyle) {
             if (context.inRightmostCompound) {
                 m_elementStyle->setAffectedByFocus();
             } else {
                 m_elementStyle->setUnique();
                 element.setChildrenOrSiblingsAffectedByFocus();
             }
         }
         return matchesFocusPseudoClass(element);
     case CSSSelector::PseudoHover:
+        if (m_mode == SharingRules)
+            return true;
         if (m_mode == ResolvingStyle) {
             if (context.inRightmostCompound) {
                 m_elementStyle->setAffectedByHover();
             } else {
                 m_elementStyle->setUnique();
                 element.setChildrenOrSiblingsAffectedByHover();
             }
         }
         if (!shouldMatchHoverOrActive(context))
             return false;
         if (InspectorInstrumentation::forcePseudoState(&element, CSSSelector::PseudoHover))
             return true;
         return element.hovered();
     case CSSSelector::PseudoActive:
+        if (m_mode == SharingRules)
+            return true;
         if (m_mode == ResolvingStyle) {
             if (context.inRightmostCompound) {
                 m_elementStyle->setAffectedByActive();
             } else {
                 m_elementStyle->setUnique();
                 element.setChildrenOrSiblingsAffectedByActive();
             }
         }
         if (!shouldMatchHoverOrActive(context))
             return false;
@@ skipping 332 matching lines @@
 }
 
 bool SelectorChecker::matchesFocusPseudoClass(const Element& element)
 {
     if (InspectorInstrumentation::forcePseudoState(const_cast<Element*>(&element), CSSSelector::PseudoFocus))
         return true;
     return element.focused() && isFrameFocused(element);
 }
 
 } // namespace blink