diff --git a/Libraries/LibWeb/HTML/Timer.cpp b/Libraries/LibWeb/HTML/Timer.cpp
index 7023a00b81d..76b9adf6af0 100644
--- a/Libraries/LibWeb/HTML/Timer.cpp
+++ b/Libraries/LibWeb/HTML/Timer.cpp
@@ -13,27 +13,26 @@ namespace Web::HTML {
 
 GC_DEFINE_ALLOCATOR(Timer);
 
-GC::Ref<Timer> Timer::create(JS::Object& window_or_worker_global_scope, i32 milliseconds, Function<void()> callback, i32 id)
+GC::Ref<Timer> Timer::create(JS::Object& window_or_worker_global_scope, i32 milliseconds, Function<void()> callback, i32 id, Repeating repeating)
 {
-    auto heap_function_callback = GC::create_function(window_or_worker_global_scope.heap(), move(callback));
-    return window_or_worker_global_scope.heap().allocate<Timer>(window_or_worker_global_scope, milliseconds, heap_function_callback, id);
+    return window_or_worker_global_scope.heap().allocate<Timer>(window_or_worker_global_scope, milliseconds, move(callback), id, repeating);
 }
 
-Timer::Timer(JS::Object& window_or_worker_global_scope, i32 milliseconds, GC::Ref<GC::Function<void()>> callback, i32 id)
+Timer::Timer(JS::Object& window_or_worker_global_scope, i32 milliseconds, Function<void()> callback, i32 id, Repeating repeating)
     : m_window_or_worker_global_scope(window_or_worker_global_scope)
-    , m_callback(move(callback))
     , m_id(id)
 {
-    m_timer = Core::Timer::create_single_shot(milliseconds, [this] {
-        m_callback->function()();
-    });
+    if (repeating == Repeating::Yes)
+        m_timer = Core::Timer::create_repeating(milliseconds, move(callback));
+    else
+        m_timer = Core::Timer::create_single_shot(milliseconds, move(callback));
 }
 
 void Timer::visit_edges(Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(m_window_or_worker_global_scope);
-    visitor.visit(m_callback);
+    visitor.visit_possible_values(m_timer->on_timeout.raw_capture_range());
 }
 
 Timer::~Timer()
@@ -51,4 +50,9 @@ void Timer::stop()
     m_timer->stop();
 }
 
+void Timer::set_callback(Function<void()> callback)
+{
+    m_timer->on_timeout = move(callback);
+}
+
 }
diff --git a/Libraries/LibWeb/HTML/Timer.h b/Libraries/LibWeb/HTML/Timer.h
index c87aac0f680..97636482fb1 100644
--- a/Libraries/LibWeb/HTML/Timer.h
+++ b/Libraries/LibWeb/HTML/Timer.h
@@ -22,20 +22,26 @@ class Timer final : public JS::Cell {
     GC_DECLARE_ALLOCATOR(Timer);
 
 public:
-    static GC::Ref<Timer> create(JS::Object&, i32 milliseconds, Function<void()> callback, i32 id);
+    enum class Repeating {
+        No,
+        Yes,
+    };
+
+    static GC::Ref<Timer> create(JS::Object&, i32 milliseconds, Function<void()> callback, i32 id, Repeating);
     virtual ~Timer() override;
 
     void start();
     void stop();
 
+    void set_callback(Function<void()>);
+
 private:
-    Timer(JS::Object& window, i32 milliseconds, GC::Ref<GC::Function<void()>> callback, i32 id);
+    Timer(JS::Object& window, i32 milliseconds, Function<void()> callback, i32 id, Repeating);
 
     virtual void visit_edges(Cell::Visitor&) override;
 
     RefPtr<Core::Timer> m_timer;
     GC::Ref<JS::Object> m_window_or_worker_global_scope;
-    GC::Ref<GC::Function<void()>> m_callback;
     i32 m_id { 0 };
 };
 
diff --git a/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.cpp b/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.cpp
index 458a72ef219..8e48d2e4deb 100644
--- a/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.cpp
+++ b/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.cpp
@@ -37,6 +37,7 @@
 #include <LibWeb/HTML/Timer.h>
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/HTML/WindowOrWorkerGlobalScope.h>
+#include <LibWeb/HTML/WorkerGlobalScope.h>
 #include <LibWeb/HighResolutionTime/Performance.h>
 #include <LibWeb/HighResolutionTime/SupportedPerformanceTypes.h>
 #include <LibWeb/IndexedDB/IDBFactory.h>
@@ -672,7 +673,7 @@ i32 WindowOrWorkerGlobalScopeMixin::run_timer_initialization_steps(TimerHandler
     // 13. Set uniqueHandle to the result of running steps after a timeout given global, "setTimeout/setInterval",
     //     timeout, and completionStep.
     //     FIXME: run_steps_after_a_timeout() needs to be updated to return a unique internal value that can be used here.
-    run_steps_after_a_timeout_impl(timeout, move(completion_step), id);
+    run_steps_after_a_timeout_impl(timeout, move(completion_step), id, repeat);
 
     // FIXME: 14. Set global's map of setTimeout and setInterval IDs[id] to uniqueHandle.
 
@@ -1074,20 +1075,29 @@ WindowOrWorkerGlobalScopeMixin::AffectedAnyWebSockets WindowOrWorkerGlobalScopeM
 // https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#run-steps-after-a-timeout
 void WindowOrWorkerGlobalScopeMixin::run_steps_after_a_timeout(i32 timeout, Function<void()> completion_step)
 {
-    return run_steps_after_a_timeout_impl(timeout, move(completion_step));
+    return run_steps_after_a_timeout_impl(timeout, move(completion_step), {}, Repeat::No);
 }
 
-void WindowOrWorkerGlobalScopeMixin::run_steps_after_a_timeout_impl(i32 timeout, Function<void()> completion_step, Optional<i32> timer_key)
+void WindowOrWorkerGlobalScopeMixin::run_steps_after_a_timeout_impl(i32 timeout, Function<void()> completion_step, Optional<i32> timer_key, Repeat repeat)
 {
     // 1. Assert: if timerKey is given, then the caller of this algorithm is the timer initialization steps. (Other specifications must not pass timerKey.)
     // Note: This is enforced by the caller.
 
-    // 2. If timerKey is not given, then set it to a new unique non-numeric value.
-    if (!timer_key.has_value())
+    // NB: We deviate from the spec here slightly by reusing existing timers if a timer_key is provided.
+    GC::Ptr<Timer> existing_timer;
+    if (timer_key.has_value()) {
+        auto result = m_timers.get(timer_key.value());
+        if (result.has_value()) {
+            existing_timer = result.value().ptr();
+            existing_timer->set_callback(move(completion_step));
+        }
+    } else {
+        // 2. If timerKey is not given, then set it to a new unique non-numeric value.
         timer_key = m_timer_id_allocator.allocate();
+    }
 
     // FIXME: 3. Let startTime be the current high resolution time given global.
-    auto timer = Timer::create(this_impl(), timeout, move(completion_step), timer_key.value());
+    auto timer = existing_timer ? GC::Ref { *existing_timer } : Timer::create(this_impl(), timeout, move(completion_step), timer_key.value(), repeat == Repeat::Yes ? Timer::Repeating::Yes : Timer::Repeating::No);
 
     // FIXME: 4. Set global's map of active timers[timerKey] to startTime plus milliseconds.
     m_timers.set(timer_key.value(), timer);
diff --git a/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.h b/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.h
index 874031136c4..c858fd7080a 100644
--- a/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.h
+++ b/Libraries/LibWeb/HTML/WindowOrWorkerGlobalScope.h
@@ -117,7 +117,7 @@ private:
         No,
     };
     i32 run_timer_initialization_steps(TimerHandler handler, i32 timeout, GC::RootVector<JS::Value> arguments, Repeat repeat, Optional<i32> previous_id = {});
-    void run_steps_after_a_timeout_impl(i32 timeout, Function<void()> completion_step, Optional<i32> timer_key = {});
+    void run_steps_after_a_timeout_impl(i32 timeout, Function<void()> completion_step, Optional<i32> timer_key, Repeat);
 
     GC::Ref<WebIDL::Promise> create_image_bitmap_impl(ImageBitmapSource& image, Optional<WebIDL::Long> sx, Optional<WebIDL::Long> sy, Optional<WebIDL::Long> sw, Optional<WebIDL::Long> sh, Optional<ImageBitmapOptions>& options) const;