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

Source/core/dom/Document.h

Index: Source/core/dom/Document.h
diff --git a/Source/core/dom/Document.h b/Source/core/dom/Document.h
index 96e20f72241a9861250a8c4c4f6b7215b9868736..2c56e601083d71dfcda5e88778b4350cb4f5179c 100644
--- a/Source/core/dom/Document.h
+++ b/Source/core/dom/Document.h
@@ -54,6 +54,7 @@
 #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"
@@ -1002,6 +1003,11 @@ public:
     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();
@@ -1366,6 +1372,8 @@ private:
     DOMTimerCoordinator m_timers;
 
     bool m_hasViewportUnits;
+    using StyleObserverSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<StyleObserver>>;
+    StyleObserverSet m_styleObservers;
 
     using DocumentVisibilityObserverSet = WillBeHeapHashSet<RawPtrWillBeWeakMember<DocumentVisibilityObserver>>;
     DocumentVisibilityObserverSet m_visibilityObservers;