diff --git a/Userland/Libraries/LibWeb/DOM/Element.cpp b/Userland/Libraries/LibWeb/DOM/Element.cpp
index 7e84527ffd8..04a71889be2 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Element.cpp
@@ -445,13 +445,13 @@ void Element::run_attribute_change_steps(FlyString const& local_name, Optional<S
     attribute_change_steps(local_name, old_value, value, namespace_);
 
     // AD-HOC: Run our own internal attribute change handler.
-    attribute_changed(local_name, value);
+    attribute_changed(local_name, old_value, value);
     invalidate_style_after_attribute_change(local_name);
 
     document().bump_dom_tree_version();
 }
 
-void Element::attribute_changed(FlyString const& name, Optional<String> const& value)
+void Element::attribute_changed(FlyString const& name, Optional<String> const&, Optional<String> const& value)
 {
     auto value_or_empty = value.value_or(String {});
 
diff --git a/Userland/Libraries/LibWeb/DOM/Element.h b/Userland/Libraries/LibWeb/DOM/Element.h
index 0092d73184d..1d92498b83e 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.h
+++ b/Userland/Libraries/LibWeb/DOM/Element.h
@@ -161,7 +161,7 @@ public:
     virtual void attribute_change_steps(FlyString const& local_name, Optional<String> const& old_value, Optional<String> const& value, Optional<FlyString> const& namespace_);
 
     void run_attribute_change_steps(FlyString const& local_name, Optional<String> const& old_value, Optional<String> const& value, Optional<FlyString> const& namespace_);
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value);
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value);
 
     CSS::RequiredInvalidationAfterStyleChange recompute_style();
 
diff --git a/Userland/Libraries/LibWeb/HTML/FormAssociatedElement.h b/Userland/Libraries/LibWeb/HTML/FormAssociatedElement.h
index 47a33e765df..61dbb83cb61 100644
--- a/Userland/Libraries/LibWeb/HTML/FormAssociatedElement.h
+++ b/Userland/Libraries/LibWeb/HTML/FormAssociatedElement.h
@@ -19,33 +19,33 @@ namespace Web::HTML {
 //    HTMLElement::inserted() -> Use form_associated_element_was_inserted()
 //    HTMLElement::removed_from() -> Use form_associated_element_was_removed()
 //
-#define FORM_ASSOCIATED_ELEMENT(ElementBaseClass, ElementClass)                                   \
-private:                                                                                          \
-    virtual HTMLElement& form_associated_element_to_html_element() override                       \
-    {                                                                                             \
-        static_assert(IsBaseOf<HTMLElement, ElementClass>);                                       \
-        return *this;                                                                             \
-    }                                                                                             \
-                                                                                                  \
-    virtual void inserted() override                                                              \
-    {                                                                                             \
-        ElementBaseClass::inserted();                                                             \
-        form_node_was_inserted();                                                                 \
-        form_associated_element_was_inserted();                                                   \
-    }                                                                                             \
-                                                                                                  \
-    virtual void removed_from(DOM::Node* node) override                                           \
-    {                                                                                             \
-        ElementBaseClass::removed_from(node);                                                     \
-        form_node_was_removed();                                                                  \
-        form_associated_element_was_removed(node);                                                \
-    }                                                                                             \
-                                                                                                  \
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override \
-    {                                                                                             \
-        ElementBaseClass::attribute_changed(name, value);                                         \
-        form_node_attribute_changed(name, value);                                                 \
-        form_associated_element_attribute_changed(name, value);                                   \
+#define FORM_ASSOCIATED_ELEMENT(ElementBaseClass, ElementClass)                                                                      \
+private:                                                                                                                             \
+    virtual HTMLElement& form_associated_element_to_html_element() override                                                          \
+    {                                                                                                                                \
+        static_assert(IsBaseOf<HTMLElement, ElementClass>);                                                                          \
+        return *this;                                                                                                                \
+    }                                                                                                                                \
+                                                                                                                                     \
+    virtual void inserted() override                                                                                                 \
+    {                                                                                                                                \
+        ElementBaseClass::inserted();                                                                                                \
+        form_node_was_inserted();                                                                                                    \
+        form_associated_element_was_inserted();                                                                                      \
+    }                                                                                                                                \
+                                                                                                                                     \
+    virtual void removed_from(DOM::Node* node) override                                                                              \
+    {                                                                                                                                \
+        ElementBaseClass::removed_from(node);                                                                                        \
+        form_node_was_removed();                                                                                                     \
+        form_associated_element_was_removed(node);                                                                                   \
+    }                                                                                                                                \
+                                                                                                                                     \
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override \
+    {                                                                                                                                \
+        ElementBaseClass::attribute_changed(name, old_value, value);                                                                 \
+        form_node_attribute_changed(name, value);                                                                                    \
+        form_associated_element_attribute_changed(name, value);                                                                      \
     }
 
 class FormAssociatedElement {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.cpp
index e872d4c563e..fcf17b51dce 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.cpp
@@ -40,9 +40,9 @@ void HTMLAnchorElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_rel_list);
 }
 
-void HTMLAnchorElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLAnchorElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
     if (name == HTML::AttributeNames::href) {
         set_the_url();
     } else if (name == HTML::AttributeNames::rel) {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.h b/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.h
index c2b6e9934a8..74c3884c6dc 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLAnchorElement.h
@@ -45,7 +45,7 @@ private:
     virtual void activation_behavior(Web::DOM::Event const&) override;
 
     // ^DOM::Element
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
     virtual i32 default_tab_index_value() const override;
 
     // ^HTML::HTMLHyperlinkElementUtils
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.cpp
index 96ceaf5a5b7..028dba9ffa5 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.cpp
@@ -33,9 +33,9 @@ void HTMLAreaElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_rel_list);
 }
 
-void HTMLAreaElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLAreaElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
     if (name == HTML::AttributeNames::href) {
         set_the_url();
     } else if (name == HTML::AttributeNames::rel) {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.h b/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.h
index 9c25c096c94..62e5170729a 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLAreaElement.h
@@ -29,7 +29,7 @@ private:
     virtual void visit_edges(Cell::Visitor&) override;
 
     // ^DOM::Element
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
     virtual i32 default_tab_index_value() const override;
 
     // ^HTML::HTMLHyperlinkElementUtils
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.cpp
index 6f1737e10e8..0484d270f68 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.cpp
@@ -46,9 +46,9 @@ void HTMLBaseElement::removed_from(Node* parent)
     document().update_base_element({});
 }
 
-void HTMLBaseElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLBaseElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
 
     // The frozen base URL must be immediately set for an element whenever any of the following situations occur:
     // - The base element is the first base element in tree order with an href content attribute in its Document, and its href content attribute is changed.
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.h b/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.h
index e2a5c44315e..3dec1001418 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLBaseElement.h
@@ -24,7 +24,7 @@ public:
 
     virtual void inserted() override;
     virtual void removed_from(Node*) override;
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
 private:
     HTMLBaseElement(DOM::Document&, DOM::QualifiedName);
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.cpp
index e558aaf6fc1..e359449d1ad 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.cpp
@@ -59,9 +59,9 @@ void HTMLBodyElement::apply_presentational_hints(CSS::StyleProperties& style) co
     });
 }
 
-void HTMLBodyElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLBodyElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
     if (name.equals_ignoring_ascii_case("link"sv)) {
         // https://html.spec.whatwg.org/multipage/rendering.html#the-page:rules-for-parsing-a-legacy-colour-value-3
         auto color = parse_legacy_color_value(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.h b/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.h
index ba172b25053..946f0f1c6de 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLBodyElement.h
@@ -21,7 +21,7 @@ class HTMLBodyElement final
 public:
     virtual ~HTMLBodyElement() override;
 
-    virtual void attribute_changed(FlyString const&, Optional<String> const&) override;
+    virtual void attribute_changed(FlyString const&, Optional<String> const& old_value, Optional<String> const&) override;
     virtual void apply_presentational_hints(CSS::StyleProperties&) const override;
 
     // https://www.w3.org/TR/html-aria/#el-body
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.cpp
index 76b99bf9e5f..0d53390f273 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.cpp
@@ -53,9 +53,9 @@ void HTMLDetailsElement::removed_from(DOM::Node*)
     set_shadow_root(nullptr);
 }
 
-void HTMLDetailsElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLDetailsElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
 
     // https://html.spec.whatwg.org/multipage/interactive-elements.html#details-notification-task-steps
     if (name == HTML::AttributeNames::open) {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.h b/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.h
index 2c4fb68b392..b98feb8cef1 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.h
@@ -34,7 +34,7 @@ private:
     virtual void inserted() override;
     virtual void removed_from(DOM::Node*) override;
     virtual void children_changed() override;
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     void queue_a_details_toggle_event_task(String old_state, String new_state);
 
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp
index aa6cd12a513..04a281d6118 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLElement.cpp
@@ -358,9 +358,9 @@ bool HTMLElement::cannot_navigate() const
     return !is<HTML::HTMLAnchorElement>(this) && !is_connected();
 }
 
-void HTMLElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Element::attribute_changed(name, value);
+    Element::attribute_changed(name, old_value, value);
 
     if (name == HTML::AttributeNames::contenteditable) {
         if (!value.has_value()) {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLElement.h b/Userland/Libraries/LibWeb/HTML/HTMLElement.h
index 09b6989d35d..230c7d85ede 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLElement.h
@@ -85,7 +85,7 @@ protected:
 
     virtual void initialize(JS::Realm&) override;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual void visit_edges(Cell::Visitor&) override;
 
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLFormElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLFormElement.cpp
index 997606e9937..ece05e94f2d 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLFormElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLFormElement.cpp
@@ -622,9 +622,9 @@ WebIDL::ExceptionOr<void> HTMLFormElement::set_action(String const& value)
     return set_attribute(AttributeNames::action, value);
 }
 
-void HTMLFormElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLFormElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
     if (name == HTML::AttributeNames::rel) {
         if (m_rel_list)
             m_rel_list->associated_attribute_changed(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLFormElement.h b/Userland/Libraries/LibWeb/HTML/HTMLFormElement.h
index 7ab8b051000..e4a13b046ba 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLFormElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLFormElement.h
@@ -111,7 +111,7 @@ private:
     virtual Vector<FlyString> supported_property_names() const override;
     virtual bool is_supported_property_index(u32) const override;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     ErrorOr<String> pick_an_encoding() const;
 
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.cpp
index 23887fd015d..f3224131d44 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.cpp
@@ -26,9 +26,9 @@ void HTMLFrameSetElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(HTMLFrameSetElement);
 }
 
-void HTMLFrameSetElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLFrameSetElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
 
 #undef __ENUMERATE
 #define __ENUMERATE(attribute_name, event_name)                     \
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.h b/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.h
index 17d32c8ae7b..43b5d73ff92 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLFrameSetElement.h
@@ -25,7 +25,7 @@ private:
     HTMLFrameSetElement(DOM::Document&, DOM::QualifiedName);
 
     virtual void initialize(JS::Realm&) override;
-    virtual void attribute_changed(FlyString const&, Optional<String> const&) override;
+    virtual void attribute_changed(FlyString const&, Optional<String> const& old_value, Optional<String> const&) override;
 
     // ^HTML::GlobalEventHandlers
     virtual JS::GCPtr<EventTarget> global_event_handlers_to_event_target(FlyString const& event_name) override;
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp
index 1828a60b0bd..91237447166 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.cpp
@@ -37,9 +37,9 @@ JS::GCPtr<Layout::Node> HTMLIFrameElement::create_layout_node(NonnullRefPtr<CSS:
     return heap().allocate_without_realm<Layout::FrameBox>(document(), *this, move(style));
 }
 
-void HTMLIFrameElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLIFrameElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
 
     // https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element:process-the-iframe-attributes-2
     // https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element:process-the-iframe-attributes-3
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h b/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h
index 237e87e7fa1..e20ece0f72f 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLIFrameElement.h
@@ -40,7 +40,7 @@ private:
     // ^DOM::Element
     virtual void inserted() override;
     virtual void removed_from(Node*) override;
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
     virtual i32 default_tab_index_value() const override;
 
     // https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element:dimension-attributes
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.cpp
index 6f7de9d79c7..01bd90e93e7 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.cpp
@@ -128,9 +128,9 @@ bool HTMLLinkElement::has_loaded_icon() const
     return m_relationship & Relationship::Icon && resource() && resource()->is_loaded() && resource()->has_encoded_data();
 }
 
-void HTMLLinkElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLLinkElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
 
     // 4.6.7 Link types - https://html.spec.whatwg.org/multipage/links.html#linkTypes
     if (name == HTML::AttributeNames::rel) {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.h b/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.h
index 276ad28d641..4f572d12265 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLLinkElement.h
@@ -47,7 +47,7 @@ private:
     HTMLLinkElement(DOM::Document&, DOM::QualifiedName);
 
     virtual void initialize(JS::Realm&) override;
-    virtual void attribute_changed(FlyString const&, Optional<String> const&) override;
+    virtual void attribute_changed(FlyString const&, Optional<String> const& old_value, Optional<String> const&) override;
 
     // ^ResourceClient
     virtual void resource_did_fail() override;
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.cpp
index a5c7a2a556f..6e7700e20de 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.cpp
@@ -98,9 +98,9 @@ void HTMLMediaElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_pending_play_promises);
 }
 
-void HTMLMediaElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLMediaElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
 
     if (name == HTML::AttributeNames::src) {
         load_element().release_value_but_fixme_should_propagate_errors();
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.h b/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.h
index 353d57beff0..d193a223b7d 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLMediaElement.h
@@ -139,7 +139,7 @@ protected:
     virtual void finalize() override;
     virtual void visit_edges(Cell::Visitor&) override;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
     virtual void removed_from(DOM::Node*) override;
     virtual void children_changed() override;
 
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.cpp
index bb1169b533d..b7e738c00e8 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.cpp
@@ -34,9 +34,9 @@ void HTMLOptionElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(HTMLOptionElement);
 }
 
-void HTMLOptionElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLOptionElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
 
     if (name == HTML::AttributeNames::selected) {
         if (!value.has_value()) {
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.h b/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.h
index 32adc64acc3..c24791ce633 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLOptionElement.h
@@ -43,7 +43,7 @@ private:
 
     virtual void initialize(JS::Realm&) override;
 
-    void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     void ask_for_a_reset();
 
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.cpp
index cfd6df6125e..4d27860f2c2 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.cpp
@@ -50,9 +50,9 @@ void HTMLScriptElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_preparation_time_document);
 }
 
-void HTMLScriptElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLScriptElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
 
     if (name == HTML::AttributeNames::crossorigin) {
         m_crossorigin = cors_setting_attribute_from_keyword(value);
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h b/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h
index 537c94d0677..44c20f975eb 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLScriptElement.h
@@ -70,7 +70,7 @@ private:
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     // https://html.spec.whatwg.org/multipage/scripting.html#prepare-the-script-element
     void prepare_script();
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLTableElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLTableElement.cpp
index 81be995ff9b..8fe9468e318 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLTableElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLTableElement.cpp
@@ -102,9 +102,9 @@ void HTMLTableElement::apply_presentational_hints(CSS::StyleProperties& style) c
     });
 }
 
-void HTMLTableElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLTableElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
     if (name == HTML::AttributeNames::cellpadding) {
         if (value.has_value())
             m_padding = max(0, parse_integer(value.value()).value_or(0));
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLTableElement.h b/Userland/Libraries/LibWeb/HTML/HTMLTableElement.h
index 04e935228b3..b60be03ab96 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLTableElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLTableElement.h
@@ -59,7 +59,7 @@ private:
     virtual void visit_edges(Cell::Visitor&) override;
 
     virtual void apply_presentational_hints(CSS::StyleProperties&) const override;
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     JS::GCPtr<DOM::HTMLCollection> mutable m_rows;
     JS::GCPtr<DOM::HTMLCollection> mutable m_t_bodies;
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.cpp
index 64af6ffe381..f2ed55bfae7 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.cpp
@@ -35,9 +35,9 @@ void HTMLTrackElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_track);
 }
 
-void HTMLTrackElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLTrackElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    HTMLElement::attribute_changed(name, value);
+    HTMLElement::attribute_changed(name, old_value, value);
 
     // https://html.spec.whatwg.org/multipage/media.html#sourcing-out-of-band-text-tracks
     // As the kind, label, and srclang attributes are set, changed, or removed, the text track must update accordingly, as per the definitions above.
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.h b/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.h
index d70606e6c0a..ae501e0b68d 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLTrackElement.h
@@ -28,7 +28,7 @@ private:
     virtual void visit_edges(Cell::Visitor&) override;
 
     // ^DOM::Element
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     JS::GCPtr<TextTrack> m_track;
 };
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.cpp b/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.cpp
index bd1b463f5b2..fb9bcd753bb 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.cpp
+++ b/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.cpp
@@ -53,9 +53,9 @@ void HTMLVideoElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_fetch_controller);
 }
 
-void HTMLVideoElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void HTMLVideoElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
 
     if (name == HTML::AttributeNames::poster) {
         determine_element_poster_frame(value).release_value_but_fixme_should_propagate_errors();
diff --git a/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.h b/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.h
index 720a78e0a04..f6d1ac9d42b 100644
--- a/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.h
+++ b/Userland/Libraries/LibWeb/HTML/HTMLVideoElement.h
@@ -49,7 +49,7 @@ private:
     virtual void finalize() override;
     virtual void visit_edges(Cell::Visitor&) override;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     // https://html.spec.whatwg.org/multipage/media.html#the-video-element:dimension-attributes
     virtual bool supports_dimension_attributes() const override { return true; }
diff --git a/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.cpp
index 07e84b30d3e..d1720d07e84 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.cpp
@@ -28,9 +28,9 @@ void SVGClipPathElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGClipPathElement);
 }
 
-void SVGClipPathElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGClipPathElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGElement::attribute_changed(name, value);
+    SVGElement::attribute_changed(name, old_value, value);
     if (name == AttributeNames::clipPathUnits)
         m_clip_path_units = AttributeParser::parse_units(value.value_or(String {}));
 }
diff --git a/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.h b/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.h
index e8782128940..41b5b51aad1 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGClipPathElement.h
@@ -33,7 +33,7 @@ public:
         return PreserveAspectRatio { PreserveAspectRatio::Align::None, {} };
     }
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     ClipPathUnits clip_path_units() const
     {
diff --git a/Userland/Libraries/LibWeb/SVG/SVGElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGElement.cpp
index de2d513ba3d..158c2db357c 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGElement.cpp
@@ -41,9 +41,9 @@ JS::NonnullGCPtr<HTML::DOMStringMap> SVGElement::dataset()
     return *m_dataset;
 }
 
-void SVGElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
 
     update_use_elements_that_reference_this();
 }
diff --git a/Userland/Libraries/LibWeb/SVG/SVGElement.h b/Userland/Libraries/LibWeb/SVG/SVGElement.h
index 8ae313fe2cb..3771a49ccc0 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGElement.h
@@ -17,7 +17,7 @@ class SVGElement : public DOM::Element {
 public:
     virtual bool requires_svg_container() const override { return true; }
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual void children_changed() override;
     virtual void inserted() override;
diff --git a/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.cpp
index 34d49a24f70..3ea426c3df5 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.cpp
@@ -25,9 +25,9 @@ void SVGEllipseElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGEllipseElement);
 }
 
-void SVGEllipseElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGEllipseElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGeometryElement::attribute_changed(name, value);
+    SVGGeometryElement::attribute_changed(name, old_value, value);
 
     if (name == SVG::AttributeNames::cx) {
         m_center_x = AttributeParser::parse_coordinate(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.h b/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.h
index a2ad882604d..60adb01cdb0 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGEllipseElement.h
@@ -18,7 +18,7 @@ class SVGEllipseElement final : public SVGGeometryElement {
 public:
     virtual ~SVGEllipseElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Gfx::Path get_path(CSSPixelSize viewport_size) override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGGradientElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGGradientElement.cpp
index 612cd9c7c9d..ebdfc1a458d 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGGradientElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGGradientElement.cpp
@@ -19,9 +19,9 @@ SVGGradientElement::SVGGradientElement(DOM::Document& document, DOM::QualifiedNa
 {
 }
 
-void SVGGradientElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGGradientElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGElement::attribute_changed(name, value);
+    SVGElement::attribute_changed(name, old_value, value);
     if (name == AttributeNames::gradientUnits) {
         m_gradient_units = AttributeParser::parse_units(value.value_or(String {}));
     } else if (name == AttributeNames::spreadMethod) {
diff --git a/Userland/Libraries/LibWeb/SVG/SVGGradientElement.h b/Userland/Libraries/LibWeb/SVG/SVGGradientElement.h
index f2beddcf8ce..99f37fb81e6 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGGradientElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGGradientElement.h
@@ -41,7 +41,7 @@ class SVGGradientElement : public SVGElement {
 public:
     virtual ~SVGGradientElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Optional<Painting::PaintStyle> to_gfx_paint_style(SVGPaintContext const&) const = 0;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.cpp
index 110d085dcc8..59a4fa3b234 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.cpp
@@ -37,9 +37,9 @@ void SVGGraphicsElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGGraphicsElement);
 }
 
-void SVGGraphicsElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGGraphicsElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGElement::attribute_changed(name, value);
+    SVGElement::attribute_changed(name, old_value, value);
     if (name == "transform"sv) {
         auto transform_list = AttributeParser::parse_transform(value.value_or(String {}));
         if (transform_list.has_value())
diff --git a/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.h b/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.h
index 95fb35eb511..bbd75eb8acc 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGGraphicsElement.h
@@ -32,7 +32,7 @@ class SVGGraphicsElement : public SVGElement {
 public:
     virtual void apply_presentational_hints(CSS::StyleProperties&) const override;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     Optional<Gfx::Color> fill_color() const;
     Optional<Gfx::Color> stroke_color() const;
diff --git a/Userland/Libraries/LibWeb/SVG/SVGLineElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGLineElement.cpp
index 125d88a647f..55e76d80ea2 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGLineElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGLineElement.cpp
@@ -25,9 +25,9 @@ void SVGLineElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGLineElement);
 }
 
-void SVGLineElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGLineElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGeometryElement::attribute_changed(name, value);
+    SVGGeometryElement::attribute_changed(name, old_value, value);
 
     if (name == SVG::AttributeNames::x1) {
         m_x1 = AttributeParser::parse_coordinate(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/SVG/SVGLineElement.h b/Userland/Libraries/LibWeb/SVG/SVGLineElement.h
index 62087b3bb63..4deef429ccc 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGLineElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGLineElement.h
@@ -18,7 +18,7 @@ class SVGLineElement final : public SVGGeometryElement {
 public:
     virtual ~SVGLineElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Gfx::Path get_path(CSSPixelSize viewport_size) override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp
index 4ee3deae913..34be130cb2f 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.cpp
@@ -28,9 +28,9 @@ void SVGLinearGradientElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGLinearGradientElement);
 }
 
-void SVGLinearGradientElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGLinearGradientElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGradientElement::attribute_changed(name, value);
+    SVGGradientElement::attribute_changed(name, old_value, value);
 
     // FIXME: Should allow for `<number-percentage> | <length>` for x1, x2, y1, y2
     if (name == SVG::AttributeNames::x1) {
diff --git a/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.h b/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.h
index fb31ff54751..eabf7cee6d3 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGLinearGradientElement.h
@@ -19,7 +19,7 @@ class SVGLinearGradientElement : public SVGGradientElement {
 public:
     virtual ~SVGLinearGradientElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Optional<Painting::PaintStyle> to_gfx_paint_style(SVGPaintContext const&) const override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGMaskElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGMaskElement.cpp
index 0104c0726ae..19eecef696a 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGMaskElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGMaskElement.cpp
@@ -33,9 +33,9 @@ JS::GCPtr<Layout::Node> SVGMaskElement::create_layout_node(NonnullRefPtr<CSS::St
     return nullptr;
 }
 
-void SVGMaskElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGMaskElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGraphicsElement::attribute_changed(name, value);
+    SVGGraphicsElement::attribute_changed(name, old_value, value);
     if (name == AttributeNames::maskUnits) {
         m_mask_units = AttributeParser::parse_units(value.value_or(String {}));
     } else if (name == AttributeNames::maskContentUnits) {
diff --git a/Userland/Libraries/LibWeb/SVG/SVGMaskElement.h b/Userland/Libraries/LibWeb/SVG/SVGMaskElement.h
index 40a11eaa355..e2a61f36d07 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGMaskElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGMaskElement.h
@@ -36,7 +36,7 @@ public:
         return PreserveAspectRatio { PreserveAspectRatio::Align::None, {} };
     }
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual JS::GCPtr<Layout::Node> create_layout_node(NonnullRefPtr<CSS::StyleProperties>) override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGPathElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGPathElement.cpp
index a65ba8a38d8..44ef4746bf7 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGPathElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGPathElement.cpp
@@ -96,9 +96,9 @@ void SVGPathElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGPathElement);
 }
 
-void SVGPathElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGPathElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGeometryElement::attribute_changed(name, value);
+    SVGGeometryElement::attribute_changed(name, old_value, value);
 
     if (name == "d")
         m_instructions = AttributeParser::parse_path_data(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/SVG/SVGPathElement.h b/Userland/Libraries/LibWeb/SVG/SVGPathElement.h
index 286b236903b..88e0d053b62 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGPathElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGPathElement.h
@@ -20,7 +20,7 @@ class SVGPathElement final : public SVGGeometryElement {
 public:
     virtual ~SVGPathElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Gfx::Path get_path(CSSPixelSize viewport_size) override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.cpp
index b667ecd4311..2c266d78ac8 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.cpp
@@ -25,9 +25,9 @@ void SVGPolygonElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGPolygonElement);
 }
 
-void SVGPolygonElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGPolygonElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGeometryElement::attribute_changed(name, value);
+    SVGGeometryElement::attribute_changed(name, old_value, value);
 
     if (name == SVG::AttributeNames::points)
         m_points = AttributeParser::parse_points(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.h b/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.h
index 5433f84c1ac..33844082469 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGPolygonElement.h
@@ -17,7 +17,7 @@ class SVGPolygonElement final : public SVGGeometryElement {
 public:
     virtual ~SVGPolygonElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Gfx::Path get_path(CSSPixelSize viewport_size) override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.cpp
index 4c600d48bee..3fab6d433c1 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.cpp
@@ -25,9 +25,9 @@ void SVGPolylineElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGPolylineElement);
 }
 
-void SVGPolylineElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGPolylineElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGeometryElement::attribute_changed(name, value);
+    SVGGeometryElement::attribute_changed(name, old_value, value);
 
     if (name == SVG::AttributeNames::points)
         m_points = AttributeParser::parse_points(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.h b/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.h
index 7c91dc16254..8bdd610a6d1 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGPolylineElement.h
@@ -17,7 +17,7 @@ class SVGPolylineElement final : public SVGGeometryElement {
 public:
     virtual ~SVGPolylineElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Gfx::Path get_path(CSSPixelSize viewport_size) override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp
index 28b2209c134..a365bb86531 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.cpp
@@ -25,9 +25,9 @@ void SVGRadialGradientElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGRadialGradientElement);
 }
 
-void SVGRadialGradientElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGRadialGradientElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGradientElement::attribute_changed(name, value);
+    SVGGradientElement::attribute_changed(name, old_value, value);
 
     // FIXME: These are <length> or <coordinate> in the spec, but all examples seem to allow percentages
     // and unitless values.
diff --git a/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.h b/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.h
index c0d43a5b4e1..8d48cbe42ef 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGRadialGradientElement.h
@@ -19,7 +19,7 @@ class SVGRadialGradientElement : public SVGGradientElement {
 public:
     virtual ~SVGRadialGradientElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Optional<Painting::PaintStyle> to_gfx_paint_style(SVGPaintContext const&) const override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGRectElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGRectElement.cpp
index f488e6d154e..e4a06f952f8 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGRectElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGRectElement.cpp
@@ -27,9 +27,9 @@ void SVGRectElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGRectElement);
 }
 
-void SVGRectElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGRectElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGeometryElement::attribute_changed(name, value);
+    SVGGeometryElement::attribute_changed(name, old_value, value);
 
     if (name == SVG::AttributeNames::x) {
         m_x = AttributeParser::parse_coordinate(value.value_or(String {}));
diff --git a/Userland/Libraries/LibWeb/SVG/SVGRectElement.h b/Userland/Libraries/LibWeb/SVG/SVGRectElement.h
index f6bc9ddfe2b..9f67ff46b6b 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGRectElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGRectElement.h
@@ -18,7 +18,7 @@ class SVGRectElement final : public SVGGeometryElement {
 public:
     virtual ~SVGRectElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual Gfx::Path get_path(CSSPixelSize viewport_size) override;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGSVGElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGSVGElement.cpp
index 3ec2d37ec7d..3515ee0f416 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGSVGElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGSVGElement.cpp
@@ -84,9 +84,9 @@ void SVGSVGElement::apply_presentational_hints(CSS::StyleProperties& style) cons
     }
 }
 
-void SVGSVGElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGSVGElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGraphicsElement::attribute_changed(name, value);
+    SVGGraphicsElement::attribute_changed(name, old_value, value);
 
     if (name.equals_ignoring_ascii_case(SVG::AttributeNames::viewBox)) {
         if (!value.has_value()) {
diff --git a/Userland/Libraries/LibWeb/SVG/SVGSVGElement.h b/Userland/Libraries/LibWeb/SVG/SVGSVGElement.h
index 9324f043889..c3b53e9b336 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGSVGElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGSVGElement.h
@@ -88,7 +88,7 @@ private:
 
     virtual bool is_svg_svg_element() const override { return true; }
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     void update_fallback_view_box_for_svg_as_image();
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGStopElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGStopElement.cpp
index 7f73ea1f42d..845ea1e6cc6 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGStopElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGStopElement.cpp
@@ -21,9 +21,9 @@ SVGStopElement::SVGStopElement(DOM::Document& document, DOM::QualifiedName quali
 {
 }
 
-void SVGStopElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGStopElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGElement::attribute_changed(name, value);
+    SVGElement::attribute_changed(name, old_value, value);
     if (name == SVG::AttributeNames::offset) {
         m_offset = AttributeParser::parse_number_percentage(value.value_or(String {}));
     }
diff --git a/Userland/Libraries/LibWeb/SVG/SVGStopElement.h b/Userland/Libraries/LibWeb/SVG/SVGStopElement.h
index be36634e243..11fd7510a97 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGStopElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGStopElement.h
@@ -20,7 +20,7 @@ class SVGStopElement final : public SVGElement {
 public:
     virtual ~SVGStopElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     JS::NonnullGCPtr<SVGAnimatedNumber> offset() const;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.cpp
index 7805361df1d..d6aa65039a8 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.cpp
@@ -42,9 +42,9 @@ void SVGSymbolElement::apply_presentational_hints(CSS::StyleProperties& style) c
     }
 }
 
-void SVGSymbolElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGSymbolElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
     if (name.equals_ignoring_ascii_case(SVG::AttributeNames::viewBox))
         m_view_box = try_parse_view_box(value.value_or(String {}));
 }
diff --git a/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.h b/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.h
index 74b2de1e5a9..30b29802d96 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGSymbolElement.h
@@ -37,7 +37,7 @@ private:
 
     bool is_direct_child_of_use_shadow_tree() const;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     Optional<ViewBox> m_view_box;
 };
diff --git a/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.cpp
index f8e0fb2d894..eecc53929fb 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.cpp
@@ -29,9 +29,9 @@ void SVGTextPositioningElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(SVGTextPositioningElement);
 }
 
-void SVGTextPositioningElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGTextPositioningElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    SVGGraphicsElement::attribute_changed(name, value);
+    SVGGraphicsElement::attribute_changed(name, old_value, value);
 
     if (name == SVG::AttributeNames::x) {
         m_x = AttributeParser::parse_coordinate(value.value_or(String {})).value_or(m_x);
diff --git a/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.h b/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.h
index f6b3788ed2d..61e86d00224 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGTextPositioningElement.h
@@ -16,7 +16,7 @@ class SVGTextPositioningElement : public SVGTextContentElement {
     WEB_PLATFORM_OBJECT(SVGTextPositioningElement, SVGTextContentElement);
 
 public:
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     Gfx::FloatPoint get_offset() const;
 
diff --git a/Userland/Libraries/LibWeb/SVG/SVGUseElement.cpp b/Userland/Libraries/LibWeb/SVG/SVGUseElement.cpp
index 4ae7ff95cd0..7b3fffebb92 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGUseElement.cpp
+++ b/Userland/Libraries/LibWeb/SVG/SVGUseElement.cpp
@@ -50,9 +50,9 @@ void SVGUseElement::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_document_observer);
 }
 
-void SVGUseElement::attribute_changed(FlyString const& name, Optional<String> const& value)
+void SVGUseElement::attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value)
 {
-    Base::attribute_changed(name, value);
+    Base::attribute_changed(name, old_value, value);
 
     // https://svgwg.org/svg2-draft/struct.html#UseLayout
     if (name == SVG::AttributeNames::x) {
diff --git a/Userland/Libraries/LibWeb/SVG/SVGUseElement.h b/Userland/Libraries/LibWeb/SVG/SVGUseElement.h
index 60371f07462..11df4d32772 100644
--- a/Userland/Libraries/LibWeb/SVG/SVGUseElement.h
+++ b/Userland/Libraries/LibWeb/SVG/SVGUseElement.h
@@ -24,7 +24,7 @@ class SVGUseElement final
 public:
     virtual ~SVGUseElement() override = default;
 
-    virtual void attribute_changed(FlyString const& name, Optional<String> const& value) override;
+    virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value) override;
 
     virtual void inserted() override;