diff --git a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
index 9890796c32..82d9ab9257 100644
--- a/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
+++ b/Userland/Libraries/LibWeb/Bindings/MainThreadVM.cpp
@@ -608,9 +608,11 @@ void queue_mutation_observer_microtask(DOM::Document const& document)
                 if (node.is_null())
                     continue;
 
-                node->registered_observers_list().remove_all_matching([&mutation_observer](DOM::RegisteredObserver& registered_observer) {
-                    return is<DOM::TransientRegisteredObserver>(registered_observer) && static_cast<DOM::TransientRegisteredObserver&>(registered_observer).observer().ptr() == mutation_observer.ptr();
-                });
+                if (node->registered_observer_list()) {
+                    node->registered_observer_list()->remove_all_matching([&mutation_observer](DOM::RegisteredObserver& registered_observer) {
+                        return is<DOM::TransientRegisteredObserver>(registered_observer) && static_cast<DOM::TransientRegisteredObserver&>(registered_observer).observer().ptr() == mutation_observer.ptr();
+                    });
+                }
             }
 
             // 4. If records is not empty, then invoke mo’s callback with « records, mo », and mo. If this throws an exception, catch it, and report the exception.
diff --git a/Userland/Libraries/LibWeb/DOM/MutationObserver.cpp b/Userland/Libraries/LibWeb/DOM/MutationObserver.cpp
index d6cf1927a0..c964fa53ab 100644
--- a/Userland/Libraries/LibWeb/DOM/MutationObserver.cpp
+++ b/Userland/Libraries/LibWeb/DOM/MutationObserver.cpp
@@ -85,26 +85,30 @@ WebIDL::ExceptionOr<void> MutationObserver::observe(Node& target, MutationObserv
 
     // 7. For each registered of target’s registered observer list, if registered’s observer is this:
     bool updated_existing_observer = false;
-    for (auto& registered_observer : target.registered_observers_list()) {
-        if (registered_observer->observer().ptr() != this)
-            continue;
-
-        updated_existing_observer = true;
-
-        // 1. For each node of this’s node list, remove all transient registered observers whose source is registered from node’s registered observer list.
-        for (auto& node : m_node_list) {
-            // FIXME: Is this correct?
-            if (node.is_null())
+    if (target.registered_observer_list()) {
+        for (auto& registered_observer : *target.registered_observer_list()) {
+            if (registered_observer->observer().ptr() != this)
                 continue;
 
-            node->registered_observers_list().remove_all_matching([&registered_observer](RegisteredObserver& observer) {
-                return is<TransientRegisteredObserver>(observer) && verify_cast<TransientRegisteredObserver>(observer).source().ptr() == registered_observer;
-            });
-        }
+            updated_existing_observer = true;
 
-        // 2. Set registered’s options to options.
-        registered_observer->set_options(options);
-        break;
+            // 1. For each node of this’s node list, remove all transient registered observers whose source is registered from node’s registered observer list.
+            for (auto& node : m_node_list) {
+                // FIXME: Is this correct?
+                if (node.is_null())
+                    continue;
+
+                if (node->registered_observer_list()) {
+                    node->registered_observer_list()->remove_all_matching([&registered_observer](RegisteredObserver& observer) {
+                        return is<TransientRegisteredObserver>(observer) && verify_cast<TransientRegisteredObserver>(observer).source().ptr() == registered_observer;
+                    });
+                }
+            }
+
+            // 2. Set registered’s options to options.
+            registered_observer->set_options(options);
+            break;
+        }
     }
 
     // 8. Otherwise:
@@ -129,9 +133,11 @@ void MutationObserver::disconnect()
         if (node.is_null())
             continue;
 
-        node->registered_observers_list().remove_all_matching([this](RegisteredObserver& registered_observer) {
-            return registered_observer.observer().ptr() == this;
-        });
+        if (node->registered_observer_list()) {
+            node->registered_observer_list()->remove_all_matching([this](RegisteredObserver& registered_observer) {
+                return registered_observer.observer().ptr() == this;
+            });
+        }
     }
 
     // 2. Empty this’s record queue.
diff --git a/Userland/Libraries/LibWeb/DOM/Node.cpp b/Userland/Libraries/LibWeb/DOM/Node.cpp
index f2616c4bc0..b43506c52d 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Node.cpp
@@ -103,8 +103,10 @@ void Node::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_layout_node);
     visitor.visit(m_paintable);
 
-    for (auto& registered_observer : m_registered_observer_list)
-        visitor.visit(registered_observer);
+    if (m_registered_observer_list) {
+        for (auto& registered_observer : *m_registered_observer_list)
+            visitor.visit(registered_observer);
+    }
 }
 
 // https://dom.spec.whatwg.org/#dom-node-baseuri
@@ -686,10 +688,12 @@ void Node::remove(bool suppress_observers)
     //     if registered’s options["subtree"] is true, then append a new transient registered observer
     //     whose observer is registered’s observer, options is registered’s options, and source is registered to node’s registered observer list.
     for (auto* inclusive_ancestor = parent; inclusive_ancestor; inclusive_ancestor = inclusive_ancestor->parent()) {
-        for (auto& registered : inclusive_ancestor->m_registered_observer_list) {
+        if (!inclusive_ancestor->m_registered_observer_list)
+            continue;
+        for (auto& registered : *inclusive_ancestor->m_registered_observer_list) {
             if (registered->options().subtree) {
                 auto transient_observer = TransientRegisteredObserver::create(registered->observer(), registered->options(), registered);
-                m_registered_observer_list.append(move(transient_observer));
+                m_registered_observer_list->append(move(transient_observer));
             }
         }
     }
@@ -1546,7 +1550,9 @@ void Node::queue_mutation_record(FlyString const& type, Optional<FlyString> cons
     // 2. Let nodes be the inclusive ancestors of target.
     // 3. For each node in nodes, and then for each registered of node’s registered observer list:
     for (auto* node = this; node; node = node->parent()) {
-        for (auto& registered_observer : node->m_registered_observer_list) {
+        if (!node->m_registered_observer_list)
+            continue;
+        for (auto& registered_observer : *node->m_registered_observer_list) {
             // 1. Let options be registered’s options.
             auto& options = registered_observer->options();
 
@@ -1995,4 +2001,11 @@ ErrorOr<void> Node::prepend_with_space(StringBuilder x, StringView const& result
     return {};
 }
 
+void Node::add_registered_observer(RegisteredObserver& registered_observer)
+{
+    if (!m_registered_observer_list)
+        m_registered_observer_list = make<Vector<JS::NonnullGCPtr<RegisteredObserver>>>();
+    m_registered_observer_list->append(registered_observer);
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/DOM/Node.h b/Userland/Libraries/LibWeb/DOM/Node.h
index 96ec472a8a..08081a6ae2 100644
--- a/Userland/Libraries/LibWeb/DOM/Node.h
+++ b/Userland/Libraries/LibWeb/DOM/Node.h
@@ -254,10 +254,10 @@ public:
 
     size_t length() const;
 
-    auto& registered_observers_list() { return m_registered_observer_list; }
-    auto const& registered_observers_list() const { return m_registered_observer_list; }
+    auto& registered_observer_list() { return m_registered_observer_list; }
+    auto const& registered_observer_list() const { return m_registered_observer_list; }
 
-    void add_registered_observer(RegisteredObserver& registered_observer) { m_registered_observer_list.append(registered_observer); }
+    void add_registered_observer(RegisteredObserver&);
 
     void queue_mutation_record(FlyString const& type, Optional<FlyString> const& attribute_name, Optional<FlyString> const& attribute_namespace, Optional<String> const& old_value, Vector<JS::Handle<Node>> added_nodes, Vector<JS::Handle<Node>> removed_nodes, Node* previous_sibling, Node* next_sibling) const;
 
@@ -693,7 +693,7 @@ protected:
 
     // https://dom.spec.whatwg.org/#registered-observer-list
     // "Nodes have a strong reference to registered observers in their registered observer list." https://dom.spec.whatwg.org/#garbage-collection
-    Vector<JS::NonnullGCPtr<RegisteredObserver>> m_registered_observer_list;
+    OwnPtr<Vector<JS::NonnullGCPtr<RegisteredObserver>>> m_registered_observer_list;
 
     void build_accessibility_tree(AccessibilityTreeNode& parent);