diff --git a/Libraries/LibWeb/Bindings/OptionConstructor.cpp b/Libraries/LibWeb/Bindings/OptionConstructor.cpp
index 3d5482f884f..601f1f4475f 100644
--- a/Libraries/LibWeb/Bindings/OptionConstructor.cpp
+++ b/Libraries/LibWeb/Bindings/OptionConstructor.cpp
@@ -77,7 +77,7 @@ JS::ThrowCompletionOr<JS::NonnullGCPtr<JS::Object>> OptionConstructor::construct
     }
 
     // 6. If selected is true, then set option's selectedness to true; otherwise set its selectedness to false (even if defaultSelected is true).
-    option_element->m_selected = vm.argument(3).to_boolean();
+    option_element->set_selected_internal(vm.argument(3).to_boolean());
 
     // 7. Return option.
     return option_element;
diff --git a/Libraries/LibWeb/HTML/HTMLOptGroupElement.cpp b/Libraries/LibWeb/HTML/HTMLOptGroupElement.cpp
index 64b568c51fd..18677ba557b 100644
--- a/Libraries/LibWeb/HTML/HTMLOptGroupElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLOptGroupElement.cpp
@@ -7,6 +7,7 @@
 #include <LibWeb/Bindings/HTMLOptGroupElementPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
 #include <LibWeb/HTML/HTMLOptGroupElement.h>
+#include <LibWeb/HTML/HTMLSelectElement.h>
 
 namespace Web::HTML {
 
@@ -25,4 +26,24 @@ void HTMLOptGroupElement::initialize(JS::Realm& realm)
     WEB_SET_PROTOTYPE_FOR_INTERFACE(HTMLOptGroupElement);
 }
 
+void HTMLOptGroupElement::inserted()
+{
+    Base::inserted();
+
+    // AD-HOC: We update the selectedness of our <select> parent here,
+    //         to ensure that the correct <option> is selected after an <optgroup> is dynamically inserted.
+    if (is<HTMLSelectElement>(*parent()) && first_child_of_type<HTMLOptionElement>())
+        static_cast<HTMLSelectElement&>(*parent()).update_selectedness();
+}
+
+void HTMLOptGroupElement::removed_from(Node* old_parent)
+{
+    Base::removed_from(old_parent);
+
+    // The optgroup HTML element removing steps, given removedNode and oldParent, are:
+    // 1. If oldParent is a select element and removedNode has an option child, then run oldParent's selectedness setting algorithm.
+    if (old_parent && is<HTMLSelectElement>(*old_parent) && first_child_of_type<HTMLOptionElement>())
+        static_cast<HTMLSelectElement&>(*old_parent).update_selectedness();
+}
+
 }
diff --git a/Libraries/LibWeb/HTML/HTMLOptGroupElement.h b/Libraries/LibWeb/HTML/HTMLOptGroupElement.h
index 4149e0dbd02..50522986f62 100644
--- a/Libraries/LibWeb/HTML/HTMLOptGroupElement.h
+++ b/Libraries/LibWeb/HTML/HTMLOptGroupElement.h
@@ -25,6 +25,8 @@ private:
     HTMLOptGroupElement(DOM::Document&, DOM::QualifiedName);
 
     virtual void initialize(JS::Realm&) override;
+    virtual void removed_from(Node*) override;
+    virtual void inserted() override;
 };
 
 }
diff --git a/Libraries/LibWeb/HTML/HTMLOptionElement.cpp b/Libraries/LibWeb/HTML/HTMLOptionElement.cpp
index 778e9a660b6..18374cd6c9d 100644
--- a/Libraries/LibWeb/HTML/HTMLOptionElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLOptionElement.cpp
@@ -1,6 +1,6 @@
 /*
  * Copyright (c) 2020, the SerenityOS developers.
- * Copyright (c) 2022, Andreas Kling <andreas@ladybird.org>
+ * Copyright (c) 2022-2024, Andreas Kling <andreas@ladybird.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -15,12 +15,15 @@
 #include <LibWeb/HTML/HTMLOptionElement.h>
 #include <LibWeb/HTML/HTMLScriptElement.h>
 #include <LibWeb/HTML/HTMLSelectElement.h>
+#include <LibWeb/HighResolutionTime/TimeOrigin.h>
 #include <LibWeb/Infra/Strings.h>
 
 namespace Web::HTML {
 
 JS_DEFINE_ALLOCATOR(HTMLOptionElement);
 
+static u64 m_next_selectedness_update_index = 1;
+
 HTMLOptionElement::HTMLOptionElement(DOM::Document& document, DOM::QualifiedName qualified_name)
     : HTMLElement(document, move(qualified_name))
 {
@@ -42,13 +45,13 @@ void HTMLOptionElement::attribute_changed(FlyString const& name, Optional<String
         if (!value.has_value()) {
             // Whenever an option element's selected attribute is removed, if its dirtiness is false, its selectedness must be set to false.
             if (!m_dirty)
-                m_selected = false;
+                set_selected_internal(false);
         } else {
             // Except where otherwise specified, when the element is created, its selectedness must be set to true
             // if the element has a selected attribute. Whenever an option element's selected attribute is added,
             // if its dirtiness is false, its selectedness must be set to true.
             if (!m_dirty)
-                m_selected = true;
+                set_selected_internal(true);
         }
     }
 }
@@ -65,6 +68,8 @@ void HTMLOptionElement::set_selected(bool selected)
 void HTMLOptionElement::set_selected_internal(bool selected)
 {
     m_selected = selected;
+    if (selected)
+        m_selectedness_update_index = m_next_selectedness_update_index++;
 }
 
 // https://html.spec.whatwg.org/multipage/form-elements.html#dom-option-value
@@ -139,7 +144,7 @@ void HTMLOptionElement::ask_for_a_reset()
 {
     // If an option element in the list of options asks for a reset, then run that select element's selectedness setting algorithm.
     if (auto* select = first_ancestor_of_type<HTMLSelectElement>()) {
-        select->update_selectedness(this);
+        select->update_selectedness();
     }
 }
 
@@ -177,4 +182,35 @@ Optional<ARIA::Role> HTMLOptionElement::default_role() const
     return ARIA::Role::option;
 }
 
+void HTMLOptionElement::inserted()
+{
+    Base::inserted();
+
+    set_selected_internal(selected());
+
+    // 1. The option HTML element insertion steps, given insertedNode, are:
+    //    If insertedNode's parent is a select element,
+    //    or insertedNode's parent is an optgroup element whose parent is a select element,
+    //    then run that select element's selectedness setting algorithm.
+    if (is<HTMLSelectElement>(*parent()))
+        static_cast<HTMLSelectElement&>(*parent()).update_selectedness();
+    else if (is<HTMLOptGroupElement>(parent()) && parent()->parent() && is<HTMLSelectElement>(*parent()->parent()))
+        static_cast<HTMLSelectElement&>(*parent()->parent()).update_selectedness();
+}
+
+void HTMLOptionElement::removed_from(Node* old_parent)
+{
+    Base::removed_from(old_parent);
+
+    // The option HTML element removing steps, given removedNode and oldParent, are:
+    // 1. If oldParent is a select element, or oldParent is an optgroup element whose parent is a select element,
+    //    then run that select element's selectedness setting algorithm.
+    if (old_parent) {
+        if (is<HTMLSelectElement>(*old_parent))
+            static_cast<HTMLSelectElement&>(*old_parent).update_selectedness();
+        else if (is<HTMLOptGroupElement>(*old_parent) && old_parent->parent_element() && is<HTMLSelectElement>(old_parent->parent_element()))
+            static_cast<HTMLSelectElement&>(*old_parent->parent_element()).update_selectedness();
+    }
+}
+
 }
diff --git a/Libraries/LibWeb/HTML/HTMLOptionElement.h b/Libraries/LibWeb/HTML/HTMLOptionElement.h
index 3481458ee20..2f176acf308 100644
--- a/Libraries/LibWeb/HTML/HTMLOptionElement.h
+++ b/Libraries/LibWeb/HTML/HTMLOptionElement.h
@@ -21,6 +21,7 @@ public:
     bool selected() const { return m_selected; }
     void set_selected(bool);
     void set_selected_internal(bool);
+    [[nodiscard]] u64 selectedness_update_index() const { return m_selectedness_update_index; }
 
     String value() const;
     WebIDL::ExceptionOr<void> set_value(String const&);
@@ -46,6 +47,9 @@ private:
 
     virtual void attribute_changed(FlyString const& name, Optional<String> const& old_value, Optional<String> const& value, Optional<FlyString> const& namespace_) override;
 
+    virtual void inserted() override;
+    virtual void removed_from(Node*) override;
+
     void ask_for_a_reset();
 
     // https://html.spec.whatwg.org/multipage/form-elements.html#concept-option-selectedness
@@ -53,6 +57,8 @@ private:
 
     // https://html.spec.whatwg.org/multipage/form-elements.html#concept-option-dirtiness
     bool m_dirty { false };
+
+    u64 m_selectedness_update_index { 0 };
 };
 
 }
diff --git a/Libraries/LibWeb/HTML/HTMLSelectElement.cpp b/Libraries/LibWeb/HTML/HTMLSelectElement.cpp
index bc48c4b551e..7f88647069f 100644
--- a/Libraries/LibWeb/HTML/HTMLSelectElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLSelectElement.cpp
@@ -211,7 +211,7 @@ void HTMLSelectElement::reset_algorithm()
     // The reset algorithm for select elements is to go through all the option elements in the element's list of options,
     for (auto const& option_element : list_of_options()) {
         // set their selectedness to true if the option element has a selected attribute, and false otherwise,
-        option_element->m_selected = option_element->has_attribute(AttributeNames::selected);
+        option_element->set_selected_internal(option_element->has_attribute(AttributeNames::selected));
         // set their dirtiness to false,
         option_element->m_dirty = false;
         // and then have the option elements ask for a reset.
@@ -241,13 +241,13 @@ void HTMLSelectElement::set_selected_index(WebIDL::Long index)
     // if any, must have its selectedness set to true and its dirtiness set to true.
     auto options = list_of_options();
     for (auto& option : options)
-        option->m_selected = false;
+        option->set_selected_internal(false);
 
     if (index < 0 || index >= static_cast<int>(options.size()))
         return;
 
     auto& selected_option = options[index];
-    selected_option->m_selected = true;
+    selected_option->set_selected_internal(true);
     selected_option->m_dirty = true;
 }
 
@@ -258,6 +258,12 @@ i32 HTMLSelectElement::default_tab_index_value() const
     return 0;
 }
 
+void HTMLSelectElement::children_changed()
+{
+    Base::children_changed();
+    update_selectedness();
+}
+
 // https://html.spec.whatwg.org/multipage/form-elements.html#dom-select-type
 String const& HTMLSelectElement::type() const
 {
@@ -562,7 +568,7 @@ void HTMLSelectElement::update_inner_text_element()
 }
 
 // https://html.spec.whatwg.org/multipage/form-elements.html#selectedness-setting-algorithm
-void HTMLSelectElement::update_selectedness(JS::GCPtr<HTML::HTMLOptionElement> last_selected_option)
+void HTMLSelectElement::update_selectedness()
 {
     if (has_attribute(AttributeNames::multiple))
         return;
@@ -605,14 +611,22 @@ void HTMLSelectElement::update_selectedness(JS::GCPtr<HTML::HTMLOptionElement> l
     if (number_of_selected >= 2) {
         // then set the selectedness of all but the last option element with its selectedness set to true
         // in the list of options in tree order to false.
+        JS::GCPtr<HTML::HTMLOptionElement> last_selected_option;
+        u64 last_selected_option_update_index = 0;
+
         for (auto const& option_element : list_of_options()) {
-            if (option_element == last_selected_option)
+            if (!option_element->selected())
                 continue;
-            if (number_of_selected == 1) {
-                break;
+            if (!last_selected_option
+                || option_element->selectedness_update_index() > last_selected_option_update_index) {
+                last_selected_option = option_element;
+                last_selected_option_update_index = option_element->selectedness_update_index();
             }
-            option_element->set_selected_internal(false);
-            --number_of_selected;
+        }
+
+        for (auto const& option_element : list_of_options()) {
+            if (option_element != last_selected_option)
+                option_element->set_selected_internal(false);
         }
     }
     update_inner_text_element();
diff --git a/Libraries/LibWeb/HTML/HTMLSelectElement.h b/Libraries/LibWeb/HTML/HTMLSelectElement.h
index d856f6606bf..91ed23160a2 100644
--- a/Libraries/LibWeb/HTML/HTMLSelectElement.h
+++ b/Libraries/LibWeb/HTML/HTMLSelectElement.h
@@ -94,7 +94,7 @@ public:
 
     void did_select_item(Optional<u32> const& id);
 
-    void update_selectedness(JS::GCPtr<HTMLOptionElement> last_selected_option = nullptr);
+    void update_selectedness();
 
 private:
     HTMLSelectElement(DOM::Document&, DOM::QualifiedName);
@@ -107,6 +107,8 @@ private:
 
     virtual void computed_css_values_changed() override;
 
+    virtual void children_changed() override;
+
     void show_the_picker_if_applicable();
 
     void create_shadow_tree_if_needed();
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-optgroup-element/optgroup-removal.window.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-optgroup-element/optgroup-removal.window.txt
index 119a10de2c7..81c743ae640 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-optgroup-element/optgroup-removal.window.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-optgroup-element/optgroup-removal.window.txt
@@ -6,6 +6,6 @@ Rerun
 
 Found 1 tests
 
-1 Fail
+1 Pass
 Details
-Result	Test Name	MessageFail	<select> needs to be updated when <optgroup> is removed	
\ No newline at end of file
+Result	Test Name	MessagePass	<select> needs to be updated when <optgroup> is removed	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/inserted-or-removed.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/inserted-or-removed.txt
index a9d81413967..057443661df 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/inserted-or-removed.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/inserted-or-removed.txt
@@ -6,11 +6,10 @@ Rerun
 
 Found 5 tests
 
-1 Pass
-4 Fail
+5 Pass
 Details
 Result	Test Name	MessagePass	The last selected OPTION should win; Inserted by parser	
-Fail	The last selected OPTION should win; Inserted by DOM API	
-Fail	The last selected OPTION should win; Inserted by jQuery append()	
-Fail	The last selected OPTION should win; Inserted by innerHTML	
-Fail	If an OPTION says it is selected, it should be selected after it is inserted.	
\ No newline at end of file
+Pass	The last selected OPTION should win; Inserted by DOM API	
+Pass	The last selected OPTION should win; Inserted by jQuery append()	
+Pass	The last selected OPTION should win; Inserted by innerHTML	
+Pass	If an OPTION says it is selected, it should be selected after it is inserted.	
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-ask-for-reset.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-ask-for-reset.txt
index f3d0e5b50db..07d093e715d 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-ask-for-reset.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-ask-for-reset.txt
@@ -6,8 +6,8 @@ Rerun
 
 Found 3 tests
 
-3 Fail
+3 Pass
 Details
-Result	Test Name	MessageFail	ask for reset on node remove, non multiple.	
-Fail	ask for reset on node insert, non multiple.	
-Fail	change selectedness of option, non multiple.	
\ No newline at end of file
+Result	Test Name	MessagePass	ask for reset on node remove, non multiple.	
+Pass	ask for reset on node insert, non multiple.	
+Pass	change selectedness of option, non multiple.	
\ No newline at end of file