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diff --git a/src/javascript/jsc/bindings/webcore/EventLoop.h b/src/javascript/jsc/bindings/webcore/EventLoop.h
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+/*
+ * Copyright (C) 2019 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
+ * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
+ * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include "TaskSource.h"
+#include <wtf/Function.h>
+#include <wtf/RefCounted.h>
+#include <wtf/StdLibExtras.h>
+#include <wtf/WeakHashSet.h>
+#include <wtf/WeakPtr.h>
+
+namespace WebCore {
+
+class ActiveDOMCallbackMicrotask;
+class EventLoopTaskGroup;
+class EventTarget;
+class MicrotaskQueue;
+class ScriptExecutionContext;
+
+class EventLoopTask {
+    WTF_MAKE_NONCOPYABLE(EventLoopTask);
+    WTF_MAKE_FAST_ALLOCATED;
+
+public:
+    virtual ~EventLoopTask() = default;
+
+    TaskSource taskSource() { return m_taskSource; }
+    virtual void execute() = 0;
+
+    EventLoopTaskGroup* group() const { return m_group.get(); }
+
+protected:
+    EventLoopTask(TaskSource, EventLoopTaskGroup&);
+
+private:
+    const TaskSource m_taskSource;
+    WeakPtr<EventLoopTaskGroup> m_group;
+};
+
+// https://html.spec.whatwg.org/multipage/webappapis.html#event-loop
+class EventLoop : public RefCounted<EventLoop>, public CanMakeWeakPtr<EventLoop> {
+public:
+    virtual ~EventLoop() = default;
+
+    typedef Function<void ()> TaskFunction;
+    void queueTask(std::unique_ptr<EventLoopTask>&&);
+
+    // https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-microtask
+    void queueMicrotask(std::unique_ptr<EventLoopTask>&&);
+
+    // https://html.spec.whatwg.org/multipage/webappapis.html#perform-a-microtask-checkpoint
+    void performMicrotaskCheckpoint();
+    virtual MicrotaskQueue& microtaskQueue() = 0;
+
+    void resumeGroup(EventLoopTaskGroup&);
+    void stopGroup(EventLoopTaskGroup&);
+
+    void registerGroup(EventLoopTaskGroup&);
+    void unregisterGroup(EventLoopTaskGroup&);
+    void stopAssociatedGroupsIfNecessary();
+
+protected:
+    EventLoop() = default;
+    void run();
+    void clearAllTasks();
+
+private:
+    void scheduleToRunIfNeeded();
+    virtual void scheduleToRun() = 0;
+    virtual bool isContextThread() const = 0;
+
+    // Use a global queue instead of multiple task queues since HTML5 spec allows UA to pick arbitrary queue.
+    Vector<std::unique_ptr<EventLoopTask>> m_tasks;
+    WeakHashSet<EventLoopTaskGroup> m_associatedGroups;
+    WeakHashSet<EventLoopTaskGroup> m_groupsWithSuspendedTasks;
+    bool m_isScheduledToRun { false };
+};
+
+class EventLoopTaskGroup : public CanMakeWeakPtr<EventLoopTaskGroup> {
+    WTF_MAKE_NONCOPYABLE(EventLoopTaskGroup);
+    WTF_MAKE_FAST_ALLOCATED;
+
+public:
+    EventLoopTaskGroup(EventLoop& eventLoop)
+        : m_eventLoop(eventLoop)
+    {
+        eventLoop.registerGroup(*this);
+    }
+
+    ~EventLoopTaskGroup()
+    {
+        if (auto* eventLoop = m_eventLoop.get())
+            eventLoop->unregisterGroup(*this);
+    }
+
+    bool hasSameEventLoopAs(EventLoopTaskGroup& otherGroup)
+    {
+        ASSERT(m_eventLoop);
+        return m_eventLoop == otherGroup.m_eventLoop;
+    }
+
+    bool matchesTask(EventLoopTask& task) const
+    {
+        auto* group = task.group();
+        return group == this;
+    }
+
+    // Marks the group as ready to stop but it won't actually be stopped
+    // until all groups in this event loop are ready to stop.
+    void markAsReadyToStop()
+    {
+        if (isReadyToStop() || isStoppedPermanently())
+            return;
+
+        bool wasSuspended = isSuspended();
+        m_state = State::ReadyToStop;
+        if (auto* eventLoop = m_eventLoop.get())
+            eventLoop->stopAssociatedGroupsIfNecessary();
+
+        if (wasSuspended && !isStoppedPermanently()) {
+            // We we get marked as ready to stop while suspended (happens when a CachedPage gets destroyed) then the
+            // queued tasks will never be able to run (since tasks don't run while suspended and we will never resume).
+            // As a result, we can simply discard our tasks and stop permanently.
+            stopAndDiscardAllTasks();
+        }
+    }
+
+    // This gets called by the event loop when all groups in the EventLoop as ready to stop.
+    void stopAndDiscardAllTasks()
+    {
+        ASSERT(isReadyToStop());
+        m_state = State::Stopped;
+        if (auto* eventLoop = m_eventLoop.get())
+            eventLoop->stopGroup(*this);
+    }
+
+    void suspend()
+    {
+        ASSERT(!isStoppedPermanently());
+        ASSERT(!isReadyToStop());
+        m_state = State::Suspended;
+        // We don't remove suspended tasks to preserve the ordering.
+        // EventLoop::run checks whether each task's group is suspended or not.
+    }
+
+    void resume()
+    {
+        ASSERT(!isStoppedPermanently());
+        ASSERT(!isReadyToStop());
+        m_state = State::Running;
+        if (auto* eventLoop = m_eventLoop.get())
+            eventLoop->resumeGroup(*this);
+    }
+
+    bool isStoppedPermanently() const { return m_state == State::Stopped; }
+    bool isSuspended() const { return m_state == State::Suspended; }
+    bool isReadyToStop() const { return m_state == State::ReadyToStop; }
+
+    void queueTask(std::unique_ptr<EventLoopTask>&&);
+    WEBCORE_EXPORT void queueTask(TaskSource, EventLoop::TaskFunction&&);
+
+    // https://html.spec.whatwg.org/multipage/webappapis.html#queue-a-microtask
+    WEBCORE_EXPORT void queueMicrotask(EventLoop::TaskFunction&&);
+    MicrotaskQueue& microtaskQueue() { return m_eventLoop->microtaskQueue(); }
+
+    // https://html.spec.whatwg.org/multipage/webappapis.html#perform-a-microtask-checkpoint
+    void performMicrotaskCheckpoint();
+
+    void runAtEndOfMicrotaskCheckpoint(EventLoop::TaskFunction&&);
+
+private:
+    enum class State : uint8_t { Running, Suspended, ReadyToStop, Stopped };
+
+    WeakPtr<EventLoop> m_eventLoop;
+    State m_state { State::Running };
+};
+
+inline EventLoopTask::EventLoopTask(TaskSource source, EventLoopTaskGroup& group)
+    : m_taskSource(source)
+    , m_group(group)
+{ }
+
+} // namespace WebCore