WIP - Rough draft of style observation in style of apply hooks.

Source/core/dom/Document.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  *           (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
@@ skipping 36 matching lines @@
 #include "core/dom/UserActionElementSet.h"
 #include "core/dom/ViewportDescription.h"
 #include "core/dom/custom/CustomElement.h"
 #include "core/fetch/ClientHintsPreferences.h"
 #include "core/frame/DOMTimerCoordinator.h"
 #include "core/frame/LocalDOMWindow.h"
 #include "core/frame/OriginsUsingFeatures.h"
 #include "core/html/CollectionType.h"
 #include "core/html/parser/ParserSynchronizationPolicy.h"
 #include "core/page/PageVisibilityState.h"
+#include "core/style/StyleObserver.h"
 #include "platform/Length.h"
 #include "platform/Timer.h"
 #include "platform/heap/Handle.h"
 #include "platform/weborigin/KURL.h"
 #include "platform/weborigin/ReferrerPolicy.h"
 #include "public/platform/WebFocusType.h"
 #include "wtf/HashSet.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/PassRefPtr.h"
@@ skipping 928 matching lines @@
 
     void updateSecurityOrigin(PassRefPtr<SecurityOrigin>);
 
     void setHasViewportUnits() { m_hasViewportUnits = true; }
     bool hasViewportUnits() const { return m_hasViewportUnits; }
     void notifyResizeForViewportUnits();
 
     void registerVisibilityObserver(DocumentVisibilityObserver*);
     void unregisterVisibilityObserver(DocumentVisibilityObserver*);
 
+    bool hasStyleObserver();
+    void notifyStyleObservers(const Element*, const ComputedStyle* oldStyle, const ComputedStyle* newStyle);
+    void registerStyleObserver(StyleObserver*);
+    void unregisterStyleObserver(StyleObserver*);
+
     void updateStyleInvalidationIfNeeded();
 
     DECLARE_VIRTUAL_TRACE();
 
     bool hasSVGFilterElementsRequiringLayerUpdate() const { return m_layerUpdateSVGFilterElements.size(); }
     void didRecalculateStyleForElement() { ++m_styleRecalcElementCounter; }
 
     AtomicString convertLocalName(const AtomicString&);
 
     void platformColorsChanged();
@@ skipping 344 matching lines @@
 
     Timer<Document> m_didAssociateFormControlsTimer;
     WillBeHeapHashSet<RefPtrWillBeMember<Element>> m_associatedFormControls;
 
     WillBeHeapHashSet<RawPtrWillBeMember<SVGUseElement>> m_useElementsNeedingUpdate;
     WillBeHeapHashSet<RawPtrWillBeMember<Element>> m_layerUpdateSVGFilterElements;
 
     DOMTimerCoordinator m_timers;
 
     bool m_hasViewportUnits;
+    using StyleObserverSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<StyleObserver>>;
+    StyleObserverSet m_styleObservers;
 
     using DocumentVisibilityObserverSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<DocumentVisibilityObserver>>;
     DocumentVisibilityObserverSet m_visibilityObservers;
 
     int m_styleRecalcElementCounter;
 
     ParserSynchronizationPolicy m_parserSyncPolicy;
 
     OriginsUsingFeatures::Value m_originsUsingFeaturesValue;
 
@@ skipping 44 matching lines @@
 DEFINE_TYPE_CASTS(TreeScope, Document, document, true, true);
 
 } // namespace blink
 
 #ifndef NDEBUG
 // Outside the WebCore namespace for ease of invocation from gdb.
 CORE_EXPORT void showLiveDocumentInstances();
 #endif
 
 #endif // Document_h