Only the main document should have a StyleEngine.

sky/engine/core/html/HTMLStyleElement.cpp

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2003, 2010 Apple Inc. All rights reserved.
  *           (C) 2007 Rob Buis (buis@kde.org)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
@@ skipping 33 matching lines @@
 HTMLStyleElement::~HTMLStyleElement()
 {
     if (m_sheet)
         m_sheet->clearOwnerNode();
 
     // TODO(esprehn): How can we still be in the document when our destructor
     // is running?
     if (inDocument()) {
         ContainerNode* scopingNode = this->scopingNode();
         TreeScope& scope = scopingNode ? scopingNode->treeScope() : treeScope();
-        document().styleEngine()->removeStyleSheetCandidateNode(this, scopingNode, scope);
+        if (StyleEngine* styleEngine = document().styleEngine())
+            styleEngine->removeStyleSheetCandidateNode(this, scopingNode, scope);
     }
 
     if (m_sheet)
         clearSheet();
 }
 
 PassRefPtr<HTMLStyleElement> HTMLStyleElement::create(Document& document)
 {
     return adoptRef(new HTMLStyleElement(document));
 }
 
 void HTMLStyleElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
     if (name == HTMLNames::mediaAttr && inDocument() && document().isActive() && m_sheet) {
         m_sheet->setMediaQueries(MediaQuerySet::create(value));
         document().modifiedStyleSheet(m_sheet.get());
     } else {
         HTMLElement::parseAttribute(name, value);
     }
 }
 
 void HTMLStyleElement::insertedInto(ContainerNode* insertionPoint)
 {
     HTMLElement::insertedInto(insertionPoint);
-    document().styleEngine()->addStyleSheetCandidateNode(this, false);
-    process();
+    if (inActiveDocument()) {
+        document().styleEngine()->addStyleSheetCandidateNode(this, false);
+        process();
+    }
 }
 
 void HTMLStyleElement::removedFrom(ContainerNode* insertionPoint)
 {
     HTMLElement::removedFrom(insertionPoint);
 
-    if (!insertionPoint->inDocument())
+    if (!insertionPoint->inActiveDocument())
         return;
 
     ShadowRoot* scopingNode = containingShadowRoot();
     if (!scopingNode)
         scopingNode = insertionPoint->containingShadowRoot();
 
     TreeScope* containingScope = containingShadowRoot();
     TreeScope& scope = containingScope ? *containingScope : insertionPoint->treeScope();
 
     document().styleEngine()->removeStyleSheetCandidateNode(this, scopingNode, scope);
@@ skipping 12 matching lines @@
     process();
 }
 
 const AtomicString& HTMLStyleElement::media() const
 {
     return getAttribute(HTMLNames::mediaAttr);
 }
 
 ContainerNode* HTMLStyleElement::scopingNode()
 {
-    if (!inDocument())
+    if (!inActiveDocument())
         return 0;
 
     if (isInShadowTree())
         return containingShadowRoot();
 
     return &document();
 }
 
 void HTMLStyleElement::clearSheet()
 {
     ASSERT(m_sheet);
     m_sheet.release()->clearOwnerNode();
 }
 
 void HTMLStyleElement::process()
 {
-    if (!inDocument())
+    if (!inActiveDocument())
         return;
 
     TRACE_EVENT0("blink", "StyleElement::process");
 
     if (m_sheet)
         clearSheet();
 
     RefPtr<MediaQuerySet> mediaQueries = MediaQuerySet::create(media());
 
     MediaQueryEvaluator screenEval("screen", true);
     if (screenEval.eval(mediaQueries.get())) {
         const String& text = textFromChildren();
         m_sheet = document().styleEngine()->createSheet(this, text);
         m_sheet->setMediaQueries(mediaQueries.release());
     }
 
     document().styleResolverChanged();
 }
 
 }