No need to look up Document from Element.

third_party/WebKit/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 506 matching lines @@
         collector.sortAndTransferMatchedRules();
         collector.finishAddingAuthorRulesForTreeScope();
     }
 
     if (!matchElementScopeDone)
         matchElementScopeRules(element, elementScopeResolver, collector);
 }
 
 void StyleResolver::matchAuthorRules(const Element& element, ElementRuleCollector& collector)
 {
-    if (element.document().styleEngine().shadowCascadeOrder() != ShadowCascadeOrder::ShadowCascadeV1) {
+    if (document().styleEngine().shadowCascadeOrder() != ShadowCascadeOrder::ShadowCascadeV1) {
         matchAuthorRulesV0(element, collector);
         return;
     }
 
     ASSERT(RuntimeEnabledFeatures::shadowDOMV1Enabled());
     matchHostRules(element, collector);
     matchScopedRules(element, collector);
 }
 
 void StyleResolver::matchAuthorRulesV0(const Element& element, ElementRuleCollector& collector)
@@ skipping 64 matching lines @@
                 state.setHasDirAutoAttribute(true);
                 collector.addElementStyleProperties(textDirection == LTR ? leftToRightDeclaration() : rightToLeftDeclaration());
             }
         }
     }
 
     matchAuthorRules(*state.element(), collector);
 
     if (state.element()->isStyledElement()) {
         // For Shadow DOM V1, inline style is already collected in matchScopedRules().
-        if (state.element()->document().styleEngine().shadowCascadeOrder() != ShadowCascadeOrder::ShadowCascadeV1 && state.element()->inlineStyle()) {
+        if (document().styleEngine().shadowCascadeOrder() != ShadowCascadeOrder::ShadowCascadeV1 && state.element()->inlineStyle()) {
             // Inline style is immutable as long as there is no CSSOM wrapper.
             bool isInlineStyleCacheable = !state.element()->inlineStyle()->isMutable();
             collector.addElementStyleProperties(state.element()->inlineStyle(), isInlineStyleCacheable);
         }
 
         // Now check SMIL animation override style.
         if (includeSMILProperties && state.element()->isSVGElement())
             collector.addElementStyleProperties(toSVGElement(state.element())->animatedSMILStyleProperties(), false /* isCacheable */);
     }
 
@@ skipping 1089 matching lines @@
     visitor->trace(m_uncommonAttributeRuleSet);
     visitor->trace(m_watchedSelectorsRules);
     visitor->trace(m_treeBoundaryCrossingScopes);
     visitor->trace(m_styleSharingLists);
     visitor->trace(m_pendingStyleSheets);
     visitor->trace(m_document);
 #endif
 }
 
 } // namespace blink