diff --git a/Libraries/LibWeb/Bindings/MainThreadVM.cpp b/Libraries/LibWeb/Bindings/MainThreadVM.cpp
index 003adef48f0..7cafd1a7b7f 100644
--- a/Libraries/LibWeb/Bindings/MainThreadVM.cpp
+++ b/Libraries/LibWeb/Bindings/MainThreadVM.cpp
@@ -31,13 +31,13 @@
 #include <LibWeb/HTML/HTMLSlotElement.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>
 #include <LibWeb/HTML/Scripting/Fetching.h>
 #include <LibWeb/HTML/Scripting/ModuleScript.h>
 #include <LibWeb/HTML/Scripting/Script.h>
+#include <LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h>
 #include <LibWeb/HTML/Scripting/SyntheticRealmSettings.h>
 #include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
 #include <LibWeb/HTML/ShadowRealmGlobalScope.h>
@@ -76,7 +76,7 @@ ErrorOr<void> initialize_main_thread_vm(HTML::EventLoop::Type type)
 {
     VERIFY(!s_main_thread_vm);
 
-    s_main_thread_vm = TRY(JS::VM::create(make<HTML::Agent>()));
+    s_main_thread_vm = TRY(JS::VM::create(make<HTML::SimilarOriginWindowAgent>()));
 
     auto& agent = as<HTML::Agent>(*s_main_thread_vm->agent());
     agent.event_loop = s_main_thread_vm->heap().allocate<HTML::EventLoop>(type);
@@ -659,7 +659,7 @@ JS::VM& main_thread_vm()
 void queue_mutation_observer_microtask(DOM::Document const& document)
 {
     auto& vm = main_thread_vm();
-    auto& surrounding_agent = as<HTML::Agent>(*vm.agent());
+    auto& surrounding_agent = as<HTML::SimilarOriginWindowAgent>(*vm.agent());
 
     // 1. If the surrounding agent’s mutation observer microtask queued is true, then return.
     if (surrounding_agent.mutation_observer_microtask_queued)
diff --git a/Libraries/LibWeb/CMakeLists.txt b/Libraries/LibWeb/CMakeLists.txt
index d57fd088114..d040a2905f8 100644
--- a/Libraries/LibWeb/CMakeLists.txt
+++ b/Libraries/LibWeb/CMakeLists.txt
@@ -504,6 +504,7 @@ set(SOURCES
     HTML/Scripting/ModuleMap.cpp
     HTML/Scripting/ModuleScript.cpp
     HTML/Scripting/Script.cpp
+    HTML/Scripting/SimilarOriginWindowAgent.cpp
     HTML/Scripting/SyntheticRealmSettings.cpp
     HTML/Scripting/TemporaryExecutionContext.cpp
     HTML/Scripting/WindowEnvironmentSettingsObject.cpp
diff --git a/Libraries/LibWeb/DOM/Element.cpp b/Libraries/LibWeb/DOM/Element.cpp
index 963e22c3789..4ad66bdb650 100644
--- a/Libraries/LibWeb/DOM/Element.cpp
+++ b/Libraries/LibWeb/DOM/Element.cpp
@@ -63,7 +63,7 @@
 #include <LibWeb/HTML/HTMLUListElement.h>
 #include <LibWeb/HTML/Numbers.h>
 #include <LibWeb/HTML/Parser/HTMLParser.h>
-#include <LibWeb/HTML/Scripting/Agent.h>
+#include <LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h>
 #include <LibWeb/HTML/Scripting/TemporaryExecutionContext.h>
 #include <LibWeb/HTML/TraversableNavigable.h>
 #include <LibWeb/HTML/Window.h>
@@ -2487,7 +2487,7 @@ bool Element::include_in_accessibility_tree() const
 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& relevant_agent = HTML::relevant_similar_origin_window_agent(*this);
     auto& reactions_stack = relevant_agent.custom_element_reactions_stack;
 
     // 2. If reactionsStack is empty, then:
@@ -2505,7 +2505,7 @@ 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(heap(), [this]() {
-            auto& reactions_stack = HTML::relevant_agent(*this).custom_element_reactions_stack;
+            auto& reactions_stack = HTML::relevant_similar_origin_window_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);
diff --git a/Libraries/LibWeb/DOM/MutationObserver.cpp b/Libraries/LibWeb/DOM/MutationObserver.cpp
index 3c19f552309..5f30c900953 100644
--- a/Libraries/LibWeb/DOM/MutationObserver.cpp
+++ b/Libraries/LibWeb/DOM/MutationObserver.cpp
@@ -9,7 +9,7 @@
 #include <LibWeb/Bindings/MutationObserverPrototype.h>
 #include <LibWeb/DOM/MutationObserver.h>
 #include <LibWeb/DOM/Node.h>
-#include <LibWeb/HTML/Scripting/Agent.h>
+#include <LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h>
 
 namespace Web::DOM {
 
@@ -30,7 +30,7 @@ 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.
-    HTML::relevant_agent(*this).mutation_observers.append(*this);
+    HTML::relevant_similar_origin_window_agent(*this).mutation_observers.append(*this);
 }
 
 MutationObserver::~MutationObserver()
@@ -39,7 +39,7 @@ MutationObserver::~MutationObserver()
 
 void MutationObserver::finalize()
 {
-    HTML::relevant_agent(*this).mutation_observers.remove(*this);
+    HTML::relevant_similar_origin_window_agent(*this).mutation_observers.remove(*this);
 }
 
 void MutationObserver::initialize(JS::Realm& realm)
diff --git a/Libraries/LibWeb/DOM/Slottable.cpp b/Libraries/LibWeb/DOM/Slottable.cpp
index d033a0eee7e..10d648c606b 100644
--- a/Libraries/LibWeb/DOM/Slottable.cpp
+++ b/Libraries/LibWeb/DOM/Slottable.cpp
@@ -11,6 +11,7 @@
 #include <LibWeb/DOM/Slottable.h>
 #include <LibWeb/DOM/Text.h>
 #include <LibWeb/HTML/HTMLSlotElement.h>
+#include <LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h>
 
 namespace Web::DOM {
 
@@ -207,7 +208,7 @@ void assign_a_slot(Slottable const& slottable)
 void signal_a_slot_change(GC::Ref<HTML::HTMLSlotElement> slottable)
 {
     // 1. Append slot to slot’s relevant agent’s signal slots.
-    HTML::relevant_agent(slottable).signal_slots.append(slottable);
+    HTML::relevant_similar_origin_window_agent(slottable).signal_slots.append(slottable);
 
     // 2. Queue a mutation observer microtask.
     Bindings::queue_mutation_observer_microtask(slottable->document());
diff --git a/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp b/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
index 144061d5856..1607c6c8fcd 100644
--- a/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
+++ b/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp
@@ -38,8 +38,8 @@
 #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/Scripting/SimilarOriginWindowAgent.h>
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/HighResolutionTime/TimeOrigin.h>
 #include <LibWeb/Infra/CharacterTypes.h>
@@ -801,7 +801,7 @@ 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.
-        relevant_agent(document).custom_element_reactions_stack.element_queue_stack.append({});
+        relevant_similar_origin_window_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.
@@ -826,7 +826,7 @@ 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 queue = relevant_agent(document).custom_element_reactions_stack.element_queue_stack.take_last();
+        auto queue = relevant_similar_origin_window_agent(document).custom_element_reactions_stack.element_queue_stack.take_last();
 
         // 2. Invoke custom element reactions in queue.
         Bindings::invoke_custom_element_reactions(queue);
@@ -5240,7 +5240,7 @@ 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) {
-        relevant_agent(*element).custom_element_reactions_stack.element_queue_stack.append({});
+        relevant_similar_origin_window_agent(*element).custom_element_reactions_stack.element_queue_stack.append({});
     }
 
     // 4. Insert element at the adjusted insertion location.
@@ -5249,7 +5249,7 @@ 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 queue = relevant_agent(*element).custom_element_reactions_stack.element_queue_stack.take_last();
+        auto queue = relevant_similar_origin_window_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
index 9a3752e4aee..f33da08bf6c 100644
--- a/Libraries/LibWeb/HTML/Scripting/Agent.cpp
+++ b/Libraries/LibWeb/HTML/Scripting/Agent.cpp
@@ -11,12 +11,6 @@
 
 namespace Web::HTML {
 
-bool Agent::can_block() const
-{
-    // similar-origin window agents can not block, see: https://html.spec.whatwg.org/multipage/webappapis.html#obtain-similar-origin-window-agent
-    return false;
-}
-
 void Agent::spin_event_loop_until(GC::Root<GC::Function<bool()>> goal_condition)
 {
     Platform::EventLoopPlugin::the().spin_until(move(goal_condition));
diff --git a/Libraries/LibWeb/HTML/Scripting/Agent.h b/Libraries/LibWeb/HTML/Scripting/Agent.h
index ccc3273403b..13303c55918 100644
--- a/Libraries/LibWeb/HTML/Scripting/Agent.h
+++ b/Libraries/LibWeb/HTML/Scripting/Agent.h
@@ -6,45 +6,20 @@
 
 #pragma once
 
-#include <AK/OwnPtr.h>
-#include <AK/Vector.h>
 #include <LibGC/Root.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Runtime/Agent.h>
-#include <LibWeb/DOM/MutationObserver.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 : public JS::Agent {
+    // https://html.spec.whatwg.org/multipage/webappapis.html#window-event-loop
+    // The event loop of a similar-origin window agent is known as a window event loop.
+    // The event loop of a dedicated worker agent, shared worker agent, or service worker agent is known as a worker event loop.
+    // And the event loop of a worklet agent is known as a worklet event loop.
     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
-    DOM::MutationObserver::List 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://dom.spec.whatwg.org/#signal-slot-list
-    // Each similar-origin window agent has signal slots (a set of slots), which is initially empty. [HTML]
-    Vector<GC::Root<HTML::HTMLSlotElement>> signal_slots;
-
-    // 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(); }
-
-    // [[CanBlock]]
-    virtual bool can_block() const override;
-
     virtual void spin_event_loop_until(GC::Root<GC::Function<bool()>> goal_condition) override;
 };
 
diff --git a/Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.cpp b/Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.cpp
new file mode 100644
index 00000000000..95e64a3016b
--- /dev/null
+++ b/Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.cpp
@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/Bindings/MainThreadVM.h>
+#include <LibWeb/HTML/Scripting/Environments.h>
+#include <LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h>
+#include <LibWeb/Platform/EventLoopPlugin.h>
+
+namespace Web::HTML {
+
+bool SimilarOriginWindowAgent::can_block() const
+{
+    // similar-origin window agents can not block, see: https://html.spec.whatwg.org/multipage/webappapis.html#obtain-similar-origin-window-agent
+    return false;
+}
+
+// https://html.spec.whatwg.org/multipage/webappapis.html#relevant-agent
+SimilarOriginWindowAgent& relevant_similar_origin_window_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 as<SimilarOriginWindowAgent>(*relevant_realm(object).vm().agent());
+}
+
+}
diff --git a/Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h b/Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h
new file mode 100644
index 00000000000..7e1ffd16b56
--- /dev/null
+++ b/Libraries/LibWeb/HTML/Scripting/SimilarOriginWindowAgent.h
@@ -0,0 +1,50 @@
+/*
+ * 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 <LibJS/Runtime/Agent.h>
+#include <LibWeb/DOM/MutationObserver.h>
+#include <LibWeb/Forward.h>
+#include <LibWeb/HTML/CustomElements/CustomElementReactionsStack.h>
+#include <LibWeb/HTML/Scripting/Agent.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/webappapis.html#similar-origin-window-agent
+struct SimilarOriginWindowAgent : public Agent {
+    // https://dom.spec.whatwg.org/#mutation-observer-compound-microtask-queued-flag
+    // Each similar-origin window agent has a mutation observer microtask queued (a boolean), which is initially false. [HTML]
+    bool mutation_observer_microtask_queued { false };
+
+    // https://dom.spec.whatwg.org/#mutation-observer-list
+    // Each similar-origin window agent also has pending mutation observers (a set of zero or more MutationObserver objects), which is initially empty.
+    DOM::MutationObserver::List 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://dom.spec.whatwg.org/#signal-slot-list
+    // Each similar-origin window agent has signal slots (a set of slots), which is initially empty. [HTML]
+    Vector<GC::Root<HTML::HTMLSlotElement>> signal_slots;
+
+    // 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(); }
+
+    // [[CanBlock]]
+    virtual bool can_block() const override;
+};
+
+SimilarOriginWindowAgent& relevant_similar_origin_window_agent(JS::Object const&);
+
+}
diff --git a/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp b/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
index 46dae457dec..e8d9e47ad30 100644
--- a/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
+++ b/Meta/Lagom/Tools/CodeGenerators/LibWeb/BindingsGenerator/IDLGenerators.cpp
@@ -2181,7 +2181,7 @@ 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& reactions_stack = HTML::relevant_agent(*impl).custom_element_reactions_stack;
+    auto& reactions_stack = HTML::relevant_similar_origin_window_agent(*impl).custom_element_reactions_stack;
     reactions_stack.element_queue_stack.append({});
 )~~~");
         }
@@ -3779,7 +3779,7 @@ 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& reactions_stack = HTML::relevant_agent(*impl).custom_element_reactions_stack;
+    auto& reactions_stack = HTML::relevant_similar_origin_window_agent(*impl).custom_element_reactions_stack;
     reactions_stack.element_queue_stack.append({});
 )~~~");
         }
@@ -4136,7 +4136,7 @@ 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& reactions_stack = HTML::relevant_agent(*impl).custom_element_reactions_stack;
+    auto& reactions_stack = HTML::relevant_similar_origin_window_agent(*impl).custom_element_reactions_stack;
     reactions_stack.element_queue_stack.append({});
 )~~~");
             }
@@ -5178,8 +5178,8 @@ 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/Scripting/SimilarOriginWindowAgent.h>
 #include <LibWeb/HTML/Window.h>
 #include <LibWeb/HTML/WindowProxy.h>
 #include <LibWeb/Infra/Strings.h>