diff --git a/Libraries/LibWeb/CSS/PseudoClasses.json b/Libraries/LibWeb/CSS/PseudoClasses.json
index d92a7fe8c48..fce688f5b8d 100644
--- a/Libraries/LibWeb/CSS/PseudoClasses.json
+++ b/Libraries/LibWeb/CSS/PseudoClasses.json
@@ -11,6 +11,9 @@
   "checked": {
     "argument": ""
   },
+  "default": {
+    "argument": ""
+  },
   "defined": {
     "argument": ""
   },
diff --git a/Libraries/LibWeb/CSS/SelectorEngine.cpp b/Libraries/LibWeb/CSS/SelectorEngine.cpp
index 79fd77662c4..cba4bbc397d 100644
--- a/Libraries/LibWeb/CSS/SelectorEngine.cpp
+++ b/Libraries/LibWeb/CSS/SelectorEngine.cpp
@@ -1037,6 +1037,29 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
 
         return false;
     }
+    case CSS::PseudoClass::Default: {
+        // https://html.spec.whatwg.org/multipage/semantics-other.html#selector-default
+
+        // The :default pseudo-class must match any element falling into one of the following categories:
+        if (auto const* form_associated_element = as_if<Web::HTML::FormAssociatedElement>(element)) {
+            // - Submit buttons that are default buttons of their form owner.
+            if (form_associated_element->is_submit_button() && form_associated_element->form() && form_associated_element->form()->default_button() == form_associated_element)
+                return true;
+
+            // - input elements to which the checked attribute applies and that have a checked attribute
+            if (auto const* input_element = as_if<Web::HTML::HTMLInputElement>(form_associated_element)) {
+                if (input_element->checked_applies() && input_element->has_attribute(HTML::AttributeNames::checked))
+                    return true;
+            }
+        }
+        // - option elements that have a selected attribute
+        else if (auto const* option_element = as_if<Web::HTML::HTMLOptionElement>(element)) {
+            if (option_element->has_attribute(HTML::AttributeNames::selected))
+                return true;
+        }
+
+        return false;
+    }
     }
 
     return false;
diff --git a/Libraries/LibWeb/CSS/StyleInvalidationData.cpp b/Libraries/LibWeb/CSS/StyleInvalidationData.cpp
index f5998387280..f8766d01059 100644
--- a/Libraries/LibWeb/CSS/StyleInvalidationData.cpp
+++ b/Libraries/LibWeb/CSS/StyleInvalidationData.cpp
@@ -75,6 +75,7 @@ static void collect_properties_used_in_has(Selector::SimpleSelector const& selec
         case PseudoClass::Link:
         case PseudoClass::AnyLink:
         case PseudoClass::LocalLink:
+        case PseudoClass::Default:
             if (in_has)
                 style_invalidation_data.pseudo_classes_used_in_has_selectors.set(pseudo_class.type);
             break;
diff --git a/Libraries/LibWeb/HTML/HTMLFormElement.cpp b/Libraries/LibWeb/HTML/HTMLFormElement.cpp
index e3fd85c6488..10f5aff1f20 100644
--- a/Libraries/LibWeb/HTML/HTMLFormElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLFormElement.cpp
@@ -1176,13 +1176,13 @@ JS::Value HTMLFormElement::named_item_value(FlyString const& name) const
 }
 
 // https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#default-button
-FormAssociatedElement* HTMLFormElement::default_button()
+FormAssociatedElement* HTMLFormElement::default_button() const
 {
     // A form element's default button is the first submit button in tree order whose form owner is that form element.
     FormAssociatedElement* default_button = nullptr;
 
     root().for_each_in_subtree([&](auto& node) {
-        auto* form_associated_element = dynamic_cast<FormAssociatedElement*>(&node);
+        auto* form_associated_element = const_cast<FormAssociatedElement*>(dynamic_cast<FormAssociatedElement const*>(&node));
         if (!form_associated_element)
             return TraversalDecision::Continue;
 
diff --git a/Libraries/LibWeb/HTML/HTMLFormElement.h b/Libraries/LibWeb/HTML/HTMLFormElement.h
index 4899dea092d..947d02caad6 100644
--- a/Libraries/LibWeb/HTML/HTMLFormElement.h
+++ b/Libraries/LibWeb/HTML/HTMLFormElement.h
@@ -102,6 +102,8 @@ public:
     String action() const;
     WebIDL::ExceptionOr<void> set_action(String const&);
 
+    FormAssociatedElement* default_button() const;
+
 private:
     HTMLFormElement(DOM::Document&, DOM::QualifiedName);
 
@@ -126,7 +128,6 @@ private:
     ErrorOr<void> mail_as_body(URL::URL parsed_action, Vector<XHR::FormDataEntry> entry_list, EncodingTypeAttributeState encoding_type, String encoding, GC::Ref<Navigable> target_navigable, Bindings::NavigationHistoryBehavior history_handling, UserNavigationInvolvement user_involvement);
     void plan_to_navigate_to(URL::URL url, Variant<Empty, String, POSTResource> post_resource, Vector<XHR::FormDataEntry> entry_list, GC::Ref<Navigable> target_navigable, Bindings::NavigationHistoryBehavior history_handling, UserNavigationInvolvement user_involvement);
 
-    FormAssociatedElement* default_button();
     size_t number_of_fields_blocking_implicit_submission() const;
 
     bool m_firing_submission_events { false };
diff --git a/Libraries/LibWeb/HTML/HTMLInputElement.cpp b/Libraries/LibWeb/HTML/HTMLInputElement.cpp
index 99b0dd25e62..36b1e35f2ac 100644
--- a/Libraries/LibWeb/HTML/HTMLInputElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLInputElement.cpp
@@ -3027,6 +3027,18 @@ bool HTMLInputElement::required_applies() const
     }
 }
 
+// https://html.spec.whatwg.org/multipage/input.html#do-not-apply
+bool HTMLInputElement::checked_applies() const
+{
+    switch (type_state()) {
+    case TypeAttributeState::Checkbox:
+    case TypeAttributeState::RadioButton:
+        return true;
+    default:
+        return false;
+    }
+}
+
 bool HTMLInputElement::has_selectable_text() const
 {
     // Potential FIXME: Date, Month, Week, Time and LocalDateAndTime are rendered as a basic text input for now,
diff --git a/Libraries/LibWeb/HTML/HTMLInputElement.h b/Libraries/LibWeb/HTML/HTMLInputElement.h
index 63e9c95c734..5cb353eaec5 100644
--- a/Libraries/LibWeb/HTML/HTMLInputElement.h
+++ b/Libraries/LibWeb/HTML/HTMLInputElement.h
@@ -220,6 +220,7 @@ public:
     bool pattern_applies() const;
     bool multiple_applies() const;
     bool required_applies() const;
+    bool checked_applies() const;
     bool has_selectable_text() const;
 
     bool supports_a_picker() const;
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/default.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/default.txt
new file mode 100644
index 00000000000..0cbeaf275fe
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/default.txt
@@ -0,0 +1,7 @@
+Harness status: OK
+
+Found 2 tests
+
+2 Pass
+Pass	':default' matches <button>s that are their form's default button, <input>s of type submit/image that are their form's default button, checked <input>s and selected <option>s
+Pass	':default' matches dynamically changed form's default buttons
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/default.html b/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/default.html
new file mode 100644
index 00000000000..0b2a656ac37
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/default.html
@@ -0,0 +1,64 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Selector: pseudo-classes (:default)</title>
+<link rel="author" title="Denis Ah-Kang" href="mailto:denis@w3.org" id=link1>
+<link rel=help href="https://html.spec.whatwg.org/multipage/#pseudo-classes" id=link2>
+<script src="../../../../resources/testharness.js"></script>
+<script src="../../../../resources/testharnessreport.js"></script>
+<script src="utils.js"></script>
+<div id="log"></div>
+<form>
+  <button id=button1 type=button>button1</button>
+  <button id=button2 type=submit>button2</button>
+</form>
+<form>
+  <button id=button3 type=reset>button3</button>
+  <button id=button4>button4</button>
+</form>
+<button id=button5 type=submit>button5</button>
+<form id=form1>
+  <input type=text id=input1>
+</form>
+<input type=text id=input2 form=form1>
+<form>
+  <input type=submit id=input3>
+  <input type=submit id=input4>
+</form>
+<form>
+  <input type=image id=input5>
+  <input type=image id=input6>
+</form>
+<form>
+  <input type=submit id=input7>
+</form>
+<input type=checkbox id=checkbox1 checked>
+<input type=checkbox id=checkbox2>
+<input type=checkbox id=checkbox3 default>
+<input type=radio name=radios id=radio1 checked>
+<input type=radio name=radios id=radio2>
+<input type=radio name=radios id=radio3 default>
+<select id=select1>
+ <optgroup label="options" id=optgroup1>
+  <option value="option1" id=option1>option1
+  <option value="option2" id=option2 selected>option2
+</select>
+<dialog id="dialog">
+  <input type=submit id=input8>
+</dialog>
+<form>
+  <button id=button6 type='invalid'>button6</button>
+  <button id=button7>button7</button>
+</form>
+<form>
+  <button id=button8>button8</button>
+  <button id=button9>button9</button>
+</form>
+
+
+<script>
+  testSelectorIdsMatch(":default", ["button2", "button4", "input3", "input5", "input7", "checkbox1", "radio1", "option2", "button6", "button8"], "':default' matches <button>s that are their form's default button, <input>s of type submit/image that are their form's default button, checked <input>s and selected <option>s");
+
+  document.getElementById("button1").type = "submit"; // change the form's default button
+  testSelectorIdsMatch(":default", ["button1", "button4", "input3", "input5", "input7", "checkbox1", "radio1", "option2", "button6", "button8"], "':default' matches dynamically changed form's default buttons");
+
+</script>