diff --git a/Libraries/LibWeb/Bindings/MainThreadVM.cpp b/Libraries/LibWeb/Bindings/MainThreadVM.cpp
index b7b894dfe6f..fc5d6655ef0 100644
--- a/Libraries/LibWeb/Bindings/MainThreadVM.cpp
+++ b/Libraries/LibWeb/Bindings/MainThreadVM.cpp
@@ -34,6 +34,7 @@
 #include <LibWeb/HTML/EventNames.h>
 #include <LibWeb/HTML/Location.h>
 #include <LibWeb/HTML/PromiseRejectionEvent.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 #include <LibWeb/HTML/Scripting/ClassicScript.h>
 #include <LibWeb/HTML/Scripting/Environments.h>
 #include <LibWeb/HTML/Scripting/ExceptionReporter.h>
@@ -92,6 +93,10 @@ ErrorOr<void> initialize_main_thread_vm(HTML::EventLoop::Type type)
     VERIFY(!s_main_thread_vm);
 
     s_main_thread_vm = TRY(JS::VM::create(make<WebEngineCustomData>()));
+
+    auto& custom_data = verify_cast<WebEngineCustomData>(*s_main_thread_vm->custom_data());
+    custom_data.agent.event_loop = s_main_thread_vm->heap().allocate<HTML::EventLoop>(type);
+
     s_main_thread_vm->on_unimplemented_property_access = [](auto const& object, auto const& property_key) {
         dbgln("FIXME: Unimplemented IDL interface: '{}.{}'", object.class_name(), property_key.to_string());
     };
@@ -100,9 +105,6 @@ ErrorOr<void> initialize_main_thread_vm(HTML::EventLoop::Type type)
     //       This avoids doing an exhaustive garbage collection on process exit.
     s_main_thread_vm->ref();
 
-    auto& custom_data = verify_cast<WebEngineCustomData>(*s_main_thread_vm->custom_data());
-    custom_data.event_loop = s_main_thread_vm->heap().allocate<HTML::EventLoop>(type);
-
     // These strings could potentially live on the VM similar to CommonPropertyNames.
     DOM::MutationType::initialize_strings();
     Editing::CommandNames::initialize_strings();
@@ -692,25 +694,24 @@ JS::VM& main_thread_vm()
 void queue_mutation_observer_microtask(DOM::Document const& document)
 {
     auto& vm = main_thread_vm();
-    auto& custom_data = verify_cast<WebEngineCustomData>(*vm.custom_data());
+    auto& surrounding_agent = verify_cast<WebEngineCustomData>(*vm.custom_data()).agent;
 
     // 1. If the surrounding agent’s mutation observer microtask queued is true, then return.
-    if (custom_data.mutation_observer_microtask_queued)
+    if (surrounding_agent.mutation_observer_microtask_queued)
         return;
 
     // 2. Set the surrounding agent’s mutation observer microtask queued to true.
-    custom_data.mutation_observer_microtask_queued = true;
+    surrounding_agent.mutation_observer_microtask_queued = true;
 
     // 3. Queue a microtask to notify mutation observers.
     // NOTE: This uses the implied document concept. In the case of mutation observers, it is always done in a node context, so document should be that node's document.
-    // FIXME: Is it safe to pass custom_data through?
-    HTML::queue_a_microtask(&document, GC::create_function(vm.heap(), [&custom_data, &heap = document.heap()]() {
+    HTML::queue_a_microtask(&document, GC::create_function(vm.heap(), [&surrounding_agent, &heap = document.heap()]() {
         // 1. Set the surrounding agent’s mutation observer microtask queued to false.
-        custom_data.mutation_observer_microtask_queued = false;
+        surrounding_agent.mutation_observer_microtask_queued = false;
 
         // 2. Let notifySet be a clone of the surrounding agent’s mutation observers.
         GC::RootVector<DOM::MutationObserver*> notify_set(heap);
-        for (auto& observer : custom_data.mutation_observers)
+        for (auto& observer : surrounding_agent.mutation_observers)
             notify_set.append(observer);
 
         // FIXME: 3. Let signalSet be a clone of the surrounding agent’s signal slots.
diff --git a/Libraries/LibWeb/Bindings/MainThreadVM.h b/Libraries/LibWeb/Bindings/MainThreadVM.h
index 97ca4ca133b..bc46a89fd66 100644
--- a/Libraries/LibWeb/Bindings/MainThreadVM.h
+++ b/Libraries/LibWeb/Bindings/MainThreadVM.h
@@ -13,56 +13,16 @@
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/DOM/MutationObserver.h>
 #include <LibWeb/HTML/EventLoop/EventLoop.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 
 namespace Web::Bindings {
 
-// https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions-stack
-struct CustomElementReactionsStack {
-    CustomElementReactionsStack() = default;
-    ~CustomElementReactionsStack() = default;
-
-    // https://html.spec.whatwg.org/multipage/custom-elements.html#element-queue
-    // Each item in the stack is an element queue, which is initially empty as well. Each item in an element queue is an element.
-    // (The elements are not necessarily custom yet, since this queue is used for upgrades as well.)
-    Vector<Vector<GC::Root<DOM::Element>>> element_queue_stack;
-
-    // https://html.spec.whatwg.org/multipage/custom-elements.html#backup-element-queue
-    // Each custom element reactions stack has an associated backup element queue, which an initially-empty element queue.
-    Vector<GC::Root<DOM::Element>> backup_element_queue;
-
-    // https://html.spec.whatwg.org/multipage/custom-elements.html#processing-the-backup-element-queue
-    // To prevent reentrancy when processing the backup element queue, each custom element reactions stack also has a processing the backup element queue flag, initially unset.
-    bool processing_the_backup_element_queue { false };
-};
-
 struct WebEngineCustomData final : public JS::VM::CustomData {
     virtual ~WebEngineCustomData() override = default;
 
     virtual void spin_event_loop_until(GC::Root<GC::Function<bool()>> goal_condition) override;
 
-    GC::Root<HTML::EventLoop> event_loop;
-
-    // FIXME: These should only be on similar-origin window agents, but we don't currently differentiate agent types.
-
-    // https://dom.spec.whatwg.org/#mutation-observer-compound-microtask-queued-flag
-    bool mutation_observer_microtask_queued { false };
-
-    // https://dom.spec.whatwg.org/#mutation-observer-list
-    // FIXME: This should be a set.
-    Vector<GC::Ref<DOM::MutationObserver>> mutation_observers;
-
-    GC::Root<JS::Realm> internal_realm;
-
-    OwnPtr<JS::ExecutionContext> root_execution_context;
-
-    // https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions-stack
-    // Each similar-origin window agent has a custom element reactions stack, which is initially empty.
-    CustomElementReactionsStack custom_element_reactions_stack {};
-
-    // https://html.spec.whatwg.org/multipage/custom-elements.html#current-element-queue
-    // A similar-origin window agent's current element queue is the element queue at the top of its custom element reactions stack.
-    Vector<GC::Root<DOM::Element>>& current_element_queue() { return custom_element_reactions_stack.element_queue_stack.last(); }
-    Vector<GC::Root<DOM::Element>> const& current_element_queue() const { return custom_element_reactions_stack.element_queue_stack.last(); }
+    HTML::Agent agent;
 };
 
 struct WebEngineCustomJobCallbackData final : public JS::JobCallback::CustomData {
diff --git a/Libraries/LibWeb/CMakeLists.txt b/Libraries/LibWeb/CMakeLists.txt
index 109bdc40524..5090decc633 100644
--- a/Libraries/LibWeb/CMakeLists.txt
+++ b/Libraries/LibWeb/CMakeLists.txt
@@ -462,6 +462,7 @@ set(SOURCES
     HTML/PotentialCORSRequest.cpp
     HTML/PromiseRejectionEvent.cpp
     HTML/RadioNodeList.cpp
+    HTML/Scripting/Agent.cpp
     HTML/Scripting/ClassicScript.cpp
     HTML/Scripting/Environments.cpp
     HTML/Scripting/EnvironmentSettingsSnapshot.cpp
diff --git a/Libraries/LibWeb/DOM/Document.cpp b/Libraries/LibWeb/DOM/Document.cpp
index feaf8926109..feea9def9b9 100644
--- a/Libraries/LibWeb/DOM/Document.cpp
+++ b/Libraries/LibWeb/DOM/Document.cpp
@@ -118,6 +118,7 @@
 #include <LibWeb/HTML/Numbers.h>
 #include <LibWeb/HTML/Parser/HTMLParser.h>
 #include <LibWeb/HTML/PopStateEvent.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 #include <LibWeb/HTML/Scripting/ClassicScript.h>
 #include <LibWeb/HTML/Scripting/ExceptionReporter.h>
 #include <LibWeb/HTML/Scripting/WindowEnvironmentSettingsObject.h>
@@ -3949,7 +3950,7 @@ void Document::unload(GC::Ptr<Document>)
     auto intend_to_store_in_bfcache = false;
 
     // 6. Let eventLoop be oldDocument's relevant agent's event loop.
-    auto& event_loop = *verify_cast<Bindings::WebEngineCustomData>(*HTML::relevant_agent(*this).custom_data()).event_loop;
+    auto& event_loop = *HTML::relevant_agent(*this).event_loop;
 
     // 7. Increase eventLoop's termination nesting level by 1.
     event_loop.increment_termination_nesting_level();
@@ -6096,7 +6097,7 @@ Document::StepsToFireBeforeunloadResult Document::steps_to_fire_beforeunload(boo
     m_unload_counter++;
 
     // 3. Increase document's relevant agent's event loop's termination nesting level by 1.
-    auto& event_loop = *verify_cast<Bindings::WebEngineCustomData>(*HTML::relevant_agent(*this).custom_data()).event_loop;
+    auto& event_loop = *HTML::relevant_agent(*this).event_loop;
     event_loop.increment_termination_nesting_level();
 
     // 4. Let eventFiringResult be the result of firing an event named beforeunload at document's relevant global object,
diff --git a/Libraries/LibWeb/DOM/Element.cpp b/Libraries/LibWeb/DOM/Element.cpp
index 1a77f1d8905..efe8ad47dff 100644
--- a/Libraries/LibWeb/DOM/Element.cpp
+++ b/Libraries/LibWeb/DOM/Element.cpp
@@ -54,6 +54,7 @@
 #include <LibWeb/HTML/HTMLTextAreaElement.h>
 #include <LibWeb/HTML/Numbers.h>
 #include <LibWeb/HTML/Parser/HTMLParser.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 #include <LibWeb/HTML/TraversableNavigable.h>
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/Infra/CharacterTypes.h>
@@ -2088,8 +2089,7 @@ void Element::enqueue_an_element_on_the_appropriate_element_queue()
 {
     // 1. Let reactionsStack be element's relevant agent's custom element reactions stack.
     auto& relevant_agent = HTML::relevant_agent(*this);
-    auto* custom_data = verify_cast<Bindings::WebEngineCustomData>(relevant_agent.custom_data());
-    auto& reactions_stack = custom_data->custom_element_reactions_stack;
+    auto& reactions_stack = relevant_agent.custom_element_reactions_stack;
 
     // 2. If reactionsStack is empty, then:
     if (reactions_stack.element_queue_stack.is_empty()) {
@@ -2105,10 +2105,8 @@ void Element::enqueue_an_element_on_the_appropriate_element_queue()
 
         // 4. Queue a microtask to perform the following steps:
         // NOTE: `this` is protected by GC::Function
-        HTML::queue_a_microtask(&document(), GC::create_function(relevant_agent.heap(), [this]() {
-            auto& relevant_agent = HTML::relevant_agent(*this);
-            auto* custom_data = verify_cast<Bindings::WebEngineCustomData>(relevant_agent.custom_data());
-            auto& reactions_stack = custom_data->custom_element_reactions_stack;
+        HTML::queue_a_microtask(&document(), GC::create_function(heap(), [this]() {
+            auto& reactions_stack = HTML::relevant_agent(*this).custom_element_reactions_stack;
 
             // 1. Invoke custom element reactions in reactionsStack's backup element queue.
             Bindings::invoke_custom_element_reactions(reactions_stack.backup_element_queue);
@@ -2121,7 +2119,7 @@ void Element::enqueue_an_element_on_the_appropriate_element_queue()
     }
 
     // 3. Otherwise, add element to element's relevant agent's current element queue.
-    custom_data->current_element_queue().append(*this);
+    relevant_agent.current_element_queue().append(*this);
 }
 
 // https://html.spec.whatwg.org/multipage/custom-elements.html#enqueue-a-custom-element-upgrade-reaction
diff --git a/Libraries/LibWeb/DOM/MutationObserver.cpp b/Libraries/LibWeb/DOM/MutationObserver.cpp
index 03a95aa830e..5393670ea59 100644
--- a/Libraries/LibWeb/DOM/MutationObserver.cpp
+++ b/Libraries/LibWeb/DOM/MutationObserver.cpp
@@ -9,6 +9,7 @@
 #include <LibWeb/Bindings/MutationObserverPrototype.h>
 #include <LibWeb/DOM/MutationObserver.h>
 #include <LibWeb/DOM/Node.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 
 namespace Web::DOM {
 
@@ -29,14 +30,12 @@ MutationObserver::MutationObserver(JS::Realm& realm, GC::Ptr<WebIDL::CallbackTyp
     // 1. Set this’s callback to callback.
 
     // 2. Append this to this’s relevant agent’s mutation observers.
-    auto* agent_custom_data = verify_cast<Bindings::WebEngineCustomData>(realm.vm().custom_data());
-    agent_custom_data->mutation_observers.append(*this);
+    HTML::relevant_agent(*this).mutation_observers.append(*this);
 }
 
 MutationObserver::~MutationObserver()
 {
-    auto* agent_custom_data = verify_cast<Bindings::WebEngineCustomData>(vm().custom_data());
-    agent_custom_data->mutation_observers.remove_all_matching([this](auto& observer) {
+    HTML::relevant_agent(*this).mutation_observers.remove_all_matching([this](auto& observer) {
         return observer.ptr() == this;
     });
 }
diff --git a/Libraries/LibWeb/Forward.h b/Libraries/LibWeb/Forward.h
index 02dae80b457..07d6692fa9e 100644
--- a/Libraries/LibWeb/Forward.h
+++ b/Libraries/LibWeb/Forward.h
@@ -552,6 +552,7 @@ enum class AllowMultipleFiles;
 enum class MediaSeekMode;
 enum class SandboxingFlagSet;
 
+struct Agent;
 struct EmbedderPolicy;
 struct Environment;
 struct EnvironmentSettingsObject;
diff --git a/Libraries/LibWeb/HTML/CustomElements/CustomElementReactionsStack.h b/Libraries/LibWeb/HTML/CustomElements/CustomElementReactionsStack.h
new file mode 100644
index 00000000000..92656cc6ecd
--- /dev/null
+++ b/Libraries/LibWeb/HTML/CustomElements/CustomElementReactionsStack.h
@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) 2021-2023, Luke Wilde <lukew@serenityos.org>
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Vector.h>
+#include <LibGC/Root.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions-stack
+struct CustomElementReactionsStack {
+    CustomElementReactionsStack() = default;
+    ~CustomElementReactionsStack() = default;
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#element-queue
+    // Each item in the stack is an element queue, which is initially empty as well. Each item in an element queue is an element.
+    // (The elements are not necessarily custom yet, since this queue is used for upgrades as well.)
+    Vector<Vector<GC::Root<DOM::Element>>> element_queue_stack;
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#backup-element-queue
+    // Each custom element reactions stack has an associated backup element queue, which an initially-empty element queue.
+    Vector<GC::Root<DOM::Element>> backup_element_queue;
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#processing-the-backup-element-queue
+    // To prevent reentrancy when processing the backup element queue, each custom element reactions stack also has a processing the backup element queue flag, initially unset.
+    bool processing_the_backup_element_queue { false };
+};
+
+}
diff --git a/Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp b/Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp
index 4a6d370f841..13cc1bca9b8 100644
--- a/Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp
+++ b/Libraries/LibWeb/HTML/EventLoop/EventLoop.cpp
@@ -14,6 +14,7 @@
 #include <LibWeb/DOM/Element.h>
 #include <LibWeb/HTML/BrowsingContext.h>
 #include <LibWeb/HTML/EventLoop/EventLoop.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 #include <LibWeb/HTML/Scripting/Environments.h>
 #include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
 #include <LibWeb/HTML/TraversableNavigable.h>
@@ -67,7 +68,7 @@ void EventLoop::schedule()
 
 EventLoop& main_thread_event_loop()
 {
-    return *static_cast<Bindings::WebEngineCustomData*>(Bindings::main_thread_vm().custom_data())->event_loop;
+    return *static_cast<Bindings::WebEngineCustomData*>(Bindings::main_thread_vm().custom_data())->agent.event_loop;
 }
 
 // https://html.spec.whatwg.org/multipage/webappapis.html#spin-the-event-loop
@@ -460,8 +461,7 @@ TaskID queue_a_task(HTML::Task::Source source, GC::Ptr<EventLoop> event_loop, GC
 TaskID queue_global_task(HTML::Task::Source source, JS::Object& global_object, GC::Ref<GC::Function<void()>> steps)
 {
     // 1. Let event loop be global's relevant agent's event loop.
-    auto& global_custom_data = verify_cast<Bindings::WebEngineCustomData>(*global_object.vm().custom_data());
-    auto& event_loop = global_custom_data.event_loop;
+    auto& event_loop = relevant_agent(global_object).event_loop;
 
     // 2. Let document be global's associated Document, if global is a Window object; otherwise null.
     DOM::Document* document { nullptr };
diff --git a/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp b/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
index 3f36c6a6ac1..be21f5b48e0 100644
--- a/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
+++ b/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
@@ -36,6 +36,7 @@
 #include <LibWeb/HTML/Parser/HTMLEncodingDetection.h>
 #include <LibWeb/HTML/Parser/HTMLParser.h>
 #include <LibWeb/HTML/Parser/HTMLToken.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 #include <LibWeb/HTML/Scripting/ExceptionReporter.h>
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/HighResolutionTime/TimeOrigin.h>
@@ -752,8 +753,7 @@ GC::Ref<DOM::Element> HTMLParser::create_element_for(HTMLToken const& token, Opt
             perform_a_microtask_checkpoint();
 
         // 3. Push a new element queue onto document's relevant agent's custom element reactions stack.
-        auto& custom_data = verify_cast<Bindings::WebEngineCustomData>(*vm.custom_data());
-        custom_data.custom_element_reactions_stack.element_queue_stack.append({});
+        relevant_agent(document).custom_element_reactions_stack.element_queue_stack.append({});
     }
 
     // 9. Let element be the result of creating an element given document, localName, given namespace, null, is, and willExecuteScript.
@@ -770,9 +770,7 @@ GC::Ref<DOM::Element> HTMLParser::create_element_for(HTMLToken const& token, Opt
     // 11. If willExecuteScript is true:
     if (will_execute_script) {
         // 1. Let queue be the result of popping from document's relevant agent's custom element reactions stack. (This will be the same element queue as was pushed above.)
-        auto& vm = main_thread_event_loop().vm();
-        auto& custom_data = verify_cast<Bindings::WebEngineCustomData>(*vm.custom_data());
-        auto queue = custom_data.custom_element_reactions_stack.element_queue_stack.take_last();
+        auto queue = relevant_agent(document).custom_element_reactions_stack.element_queue_stack.take_last();
 
         // 2. Invoke custom element reactions in queue.
         Bindings::invoke_custom_element_reactions(queue);
@@ -5150,8 +5148,7 @@ void HTMLParser::insert_an_element_at_the_adjusted_insertion_location(GC::Ref<DO
     // 3. If the parser was not created as part of the HTML fragment parsing algorithm,
     //    then push a new element queue onto element's relevant agent's custom element reactions stack.
     if (!m_parsing_fragment) {
-        auto& custom_data = verify_cast<Bindings::WebEngineCustomData>(*relevant_agent(*element).custom_data());
-        custom_data.custom_element_reactions_stack.element_queue_stack.append({});
+        relevant_agent(*element).custom_element_reactions_stack.element_queue_stack.append({});
     }
 
     // 4. Insert element at the adjusted insertion location.
@@ -5160,8 +5157,7 @@ void HTMLParser::insert_an_element_at_the_adjusted_insertion_location(GC::Ref<DO
     // 5. If the parser was not created as part of the HTML fragment parsing algorithm,
     //    then pop the element queue from element's relevant agent's custom element reactions stack, and invoke custom element reactions in that queue.
     if (!m_parsing_fragment) {
-        auto& custom_data = verify_cast<Bindings::WebEngineCustomData>(*relevant_agent(*element).custom_data());
-        auto queue = custom_data.custom_element_reactions_stack.element_queue_stack.take_last();
+        auto queue = relevant_agent(*element).custom_element_reactions_stack.element_queue_stack.take_last();
         Bindings::invoke_custom_element_reactions(queue);
     }
 }
diff --git a/Libraries/LibWeb/HTML/Scripting/Agent.cpp b/Libraries/LibWeb/HTML/Scripting/Agent.cpp
new file mode 100644
index 00000000000..d148f7ff718
--- /dev/null
+++ b/Libraries/LibWeb/HTML/Scripting/Agent.cpp
@@ -0,0 +1,21 @@
+/*
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
+#include <LibWeb/HTML/Scripting/Environments.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/webappapis.html#relevant-agent
+Agent& relevant_agent(JS::Object const& object)
+{
+    // The relevant agent for a platform object platformObject is platformObject's relevant Realm's agent.
+    // Spec Note: This pointer is not yet defined in the JavaScript specification; see tc39/ecma262#1357.
+    return verify_cast<Bindings::WebEngineCustomData>(relevant_realm(object).vm().custom_data())->agent;
+}
+
+}
diff --git a/Libraries/LibWeb/HTML/Scripting/Agent.h b/Libraries/LibWeb/HTML/Scripting/Agent.h
new file mode 100644
index 00000000000..4a69bbda1bb
--- /dev/null
+++ b/Libraries/LibWeb/HTML/Scripting/Agent.h
@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/OwnPtr.h>
+#include <AK/Vector.h>
+#include <LibGC/Root.h>
+#include <LibJS/Forward.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/HTML/CustomElements/CustomElementReactionsStack.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/webappapis.html#similar-origin-window-agent
+struct Agent {
+    GC::Root<HTML::EventLoop> event_loop;
+
+    // FIXME: These should only be on similar-origin window agents, but we don't currently differentiate agent types.
+
+    // https://dom.spec.whatwg.org/#mutation-observer-compound-microtask-queued-flag
+    bool mutation_observer_microtask_queued { false };
+
+    // https://dom.spec.whatwg.org/#mutation-observer-list
+    // FIXME: This should be a set.
+    Vector<GC::Root<DOM::MutationObserver>> mutation_observers;
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#custom-element-reactions-stack
+    // Each similar-origin window agent has a custom element reactions stack, which is initially empty.
+    CustomElementReactionsStack custom_element_reactions_stack {};
+
+    // https://html.spec.whatwg.org/multipage/custom-elements.html#current-element-queue
+    // A similar-origin window agent's current element queue is the element queue at the top of its custom element reactions stack.
+    Vector<GC::Root<DOM::Element>>& current_element_queue() { return custom_element_reactions_stack.element_queue_stack.last(); }
+    Vector<GC::Root<DOM::Element>> const& current_element_queue() const { return custom_element_reactions_stack.element_queue_stack.last(); }
+};
+
+Agent& relevant_agent(JS::Object const&);
+
+}
diff --git a/Libraries/LibWeb/HTML/Scripting/Environments.cpp b/Libraries/LibWeb/HTML/Scripting/Environments.cpp
index 217711cc0b7..06634c6de90 100644
--- a/Libraries/LibWeb/HTML/Scripting/Environments.cpp
+++ b/Libraries/LibWeb/HTML/Scripting/Environments.cpp
@@ -13,6 +13,7 @@
 #include <LibWeb/DOM/Document.h>
 #include <LibWeb/DOMURL/DOMURL.h>
 #include <LibWeb/Fetch/Infrastructure/FetchRecord.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 #include <LibWeb/HTML/Scripting/Environments.h>
 #include <LibWeb/HTML/Scripting/ExceptionReporter.h>
 #include <LibWeb/HTML/Scripting/WindowEnvironmentSettingsObject.h>
@@ -102,10 +103,8 @@ EventLoop& EnvironmentSettingsObject::responsible_event_loop()
     if (m_responsible_event_loop)
         return *m_responsible_event_loop;
 
-    auto& vm = global_object().vm();
-    auto& event_loop = verify_cast<Bindings::WebEngineCustomData>(vm.custom_data())->event_loop;
-    m_responsible_event_loop = event_loop;
-    return *event_loop;
+    m_responsible_event_loop = relevant_agent(global_object()).event_loop;
+    return *m_responsible_event_loop;
 }
 
 // https://html.spec.whatwg.org/multipage/webappapis.html#check-if-we-can-run-script
@@ -513,13 +512,6 @@ JS::Object& entry_global_object()
     return entry_realm().global_object();
 }
 
-JS::VM& relevant_agent(JS::Object const& object)
-{
-    // The relevant agent for a platform object platformObject is platformObject's relevant Realm's agent.
-    // Spec Note: This pointer is not yet defined in the JavaScript specification; see tc39/ecma262#1357.
-    return relevant_realm(object).vm();
-}
-
 // https://html.spec.whatwg.org/multipage/webappapis.html#secure-context
 bool is_secure_context(Environment const& environment)
 {
diff --git a/Libraries/LibWeb/HTML/Scripting/Environments.h b/Libraries/LibWeb/HTML/Scripting/Environments.h
index d7adc8d6c74..532d1dc36d7 100644
--- a/Libraries/LibWeb/HTML/Scripting/Environments.h
+++ b/Libraries/LibWeb/HTML/Scripting/Environments.h
@@ -168,7 +168,6 @@ JS::Object& relevant_principal_global_object(JS::Object const&);
 JS::Realm& entry_realm();
 EnvironmentSettingsObject& entry_settings_object();
 JS::Object& entry_global_object();
-JS::VM& relevant_agent(JS::Object const&);
 [[nodiscard]] bool is_secure_context(Environment const&);
 [[nodiscard]] bool is_non_secure_context(Environment const&);
 
diff --git a/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp b/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
index 1b97d142a12..07332287523 100644
--- a/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
@@ -2077,9 +2077,7 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@function.name:snakecase@@overload_suffi
         if (function.extended_attributes.contains("CEReactions")) {
             // 1. Push a new element queue onto this object's relevant agent's custom element reactions stack.
             function_generator.append(R"~~~(
-    auto& relevant_agent = HTML::relevant_agent(*impl);
-    auto* custom_data = verify_cast<Bindings::WebEngineCustomData>(relevant_agent.custom_data());
-    auto& reactions_stack = custom_data->custom_element_reactions_stack;
+    auto& reactions_stack = HTML::relevant_agent(*impl).custom_element_reactions_stack;
     reactions_stack.element_queue_stack.append({});
 )~~~");
         }
@@ -3566,9 +3564,7 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.getter_callback@)
         if (attribute.extended_attributes.contains("CEReactions")) {
             // 1. Push a new element queue onto this object's relevant agent's custom element reactions stack.
             attribute_generator.append(R"~~~(
-    auto& relevant_agent = HTML::relevant_agent(*impl);
-    auto* custom_data = verify_cast<Bindings::WebEngineCustomData>(relevant_agent.custom_data());
-    auto& reactions_stack = custom_data->custom_element_reactions_stack;
+    auto& reactions_stack = HTML::relevant_agent(*impl).custom_element_reactions_stack;
     reactions_stack.element_queue_stack.append({});
 )~~~");
         }
@@ -3914,9 +3910,7 @@ JS_DEFINE_NATIVE_FUNCTION(@class_name@::@attribute.setter_callback@)
             if (attribute.extended_attributes.contains("CEReactions")) {
                 // 1. Push a new element queue onto this object's relevant agent's custom element reactions stack.
                 attribute_generator.append(R"~~~(
-    auto& relevant_agent = HTML::relevant_agent(*impl);
-    auto* custom_data = verify_cast<Bindings::WebEngineCustomData>(relevant_agent.custom_data());
-    auto& reactions_stack = custom_data->custom_element_reactions_stack;
+    auto& reactions_stack = HTML::relevant_agent(*impl).custom_element_reactions_stack;
     reactions_stack.element_queue_stack.append({});
 )~~~");
             }
@@ -4912,6 +4906,7 @@ void generate_prototype_implementation(IDL::Interface const& interface, StringBu
 #include <LibWeb/DOM/NodeFilter.h>
 #include <LibWeb/DOM/Range.h>
 #include <LibWeb/HTML/Numbers.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
 #include <LibWeb/HTML/Scripting/Environments.h>
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/HTML/WindowProxy.h>