diff --git a/Libraries/LibWeb/CSS/PseudoClasses.json b/Libraries/LibWeb/CSS/PseudoClasses.json
index b91702cc9e9..d92a7fe8c48 100644
--- a/Libraries/LibWeb/CSS/PseudoClasses.json
+++ b/Libraries/LibWeb/CSS/PseudoClasses.json
@@ -116,6 +116,9 @@
   "optimal-value": {
     "argument": ""
   },
+  "optional": {
+    "argument": ""
+  },
   "popover-open": {
     "argument": ""
   },
@@ -134,6 +137,9 @@
   "read-write": {
     "argument": ""
   },
+  "required": {
+    "argument": ""
+  },
   "root": {
     "argument": ""
   },
diff --git a/Libraries/LibWeb/CSS/SelectorEngine.cpp b/Libraries/LibWeb/CSS/SelectorEngine.cpp
index 963131dd556..79fd77662c4 100644
--- a/Libraries/LibWeb/CSS/SelectorEngine.cpp
+++ b/Libraries/LibWeb/CSS/SelectorEngine.cpp
@@ -988,6 +988,55 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
 
         return false;
     }
+    case CSS::PseudoClass::Required: {
+        // https://html.spec.whatwg.org/multipage/semantics-other.html#selector-required
+
+        // The :required pseudo-class must match any element falling into one of the following categories:
+        // - input elements that are required
+        if (auto const* input_element = as_if<Web::HTML::HTMLInputElement>(element)) {
+            if (input_element->required_applies() && input_element->has_attribute(HTML::AttributeNames::required))
+                return true;
+        }
+        // - select elements that have a required attribute
+        else if (auto const* select_element = as_if<Web::HTML::HTMLSelectElement>(element)) {
+            if (select_element->has_attribute(HTML::AttributeNames::required))
+                return true;
+        }
+        // - textarea elements that have a required attribute
+        else if (auto const* textarea_element = as_if<Web::HTML::HTMLTextAreaElement>(element)) {
+            if (textarea_element->has_attribute(HTML::AttributeNames::required))
+                return true;
+        }
+
+        return false;
+    }
+    case CSS::PseudoClass::Optional: {
+        // https://html.spec.whatwg.org/multipage/semantics-other.html#selector-optional
+
+        // The :optional pseudo-class must match any element falling into one of the following categories:
+        // - input elements to which the required attribute applies that are not required
+        if (auto const* input_element = as_if<Web::HTML::HTMLInputElement>(element)) {
+            if (input_element->required_applies() && !input_element->has_attribute(HTML::AttributeNames::required))
+                return true;
+
+            // AD-HOC: Chromium and Webkit also match for hidden inputs (and WPT expects this)
+            // See: https://github.com/whatwg/html/issues/11273
+            if (input_element->type_state() == HTML::HTMLInputElement::TypeAttributeState::Hidden)
+                return true;
+        }
+        // - select elements that do not have a required attribute
+        else if (auto const* select_element = as_if<Web::HTML::HTMLSelectElement>(element)) {
+            if (!select_element->has_attribute(HTML::AttributeNames::required))
+                return true;
+        }
+        // - textarea elements that do not have a required attribute
+        else if (auto const* textarea_element = as_if<Web::HTML::HTMLTextAreaElement>(element)) {
+            if (!textarea_element->has_attribute(HTML::AttributeNames::required))
+                return true;
+        }
+
+        return false;
+    }
     }
 
     return false;
diff --git a/Libraries/LibWeb/CSS/StyleInvalidationData.cpp b/Libraries/LibWeb/CSS/StyleInvalidationData.cpp
index 27865e2d6bc..f5998387280 100644
--- a/Libraries/LibWeb/CSS/StyleInvalidationData.cpp
+++ b/Libraries/LibWeb/CSS/StyleInvalidationData.cpp
@@ -141,6 +141,8 @@ static void build_invalidation_sets_for_simple_selector(Selector::SimpleSelector
         case PseudoClass::Link:
         case PseudoClass::AnyLink:
         case PseudoClass::LocalLink:
+        case PseudoClass::Required:
+        case PseudoClass::Optional:
             invalidation_set.set_needs_invalidate_pseudo_class(pseudo_class.type);
             break;
         default:
diff --git a/Libraries/LibWeb/DOM/Element.cpp b/Libraries/LibWeb/DOM/Element.cpp
index e3352434fee..339fd38abcc 100644
--- a/Libraries/LibWeb/DOM/Element.cpp
+++ b/Libraries/LibWeb/DOM/Element.cpp
@@ -2397,6 +2397,9 @@ void Element::invalidate_style_after_attribute_change(FlyString const& attribute
         changed_properties.append({ .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::PlaceholderShown });
     } else if (attribute_name == HTML::AttributeNames::value) {
         changed_properties.append({ .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::Checked });
+    } else if (attribute_name == HTML::AttributeNames::required) {
+        changed_properties.append({ .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::Required });
+        changed_properties.append({ .type = CSS::InvalidationSet::Property::Type::PseudoClass, .value = CSS::PseudoClass::Optional });
     }
 
     changed_properties.append({ .type = CSS::InvalidationSet::Property::Type::Attribute, .value = attribute_name });
diff --git a/Libraries/LibWeb/HTML/HTMLInputElement.cpp b/Libraries/LibWeb/HTML/HTMLInputElement.cpp
index 2d43f712757..99b0dd25e62 100644
--- a/Libraries/LibWeb/HTML/HTMLInputElement.cpp
+++ b/Libraries/LibWeb/HTML/HTMLInputElement.cpp
@@ -3002,6 +3002,31 @@ bool HTMLInputElement::multiple_applies() const
     }
 }
 
+// https://html.spec.whatwg.org/multipage/input.html#do-not-apply
+bool HTMLInputElement::required_applies() const
+{
+    switch (type_state()) {
+    case TypeAttributeState::Text:
+    case TypeAttributeState::Search:
+    case TypeAttributeState::Telephone:
+    case TypeAttributeState::URL:
+    case TypeAttributeState::Email:
+    case TypeAttributeState::Password:
+    case TypeAttributeState::Date:
+    case TypeAttributeState::Month:
+    case TypeAttributeState::Week:
+    case TypeAttributeState::Time:
+    case TypeAttributeState::LocalDateAndTime:
+    case TypeAttributeState::Number:
+    case TypeAttributeState::Checkbox:
+    case TypeAttributeState::RadioButton:
+    case TypeAttributeState::FileUpload:
+        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 d926d1e07ca..63e9c95c734 100644
--- a/Libraries/LibWeb/HTML/HTMLInputElement.h
+++ b/Libraries/LibWeb/HTML/HTMLInputElement.h
@@ -219,6 +219,7 @@ public:
     bool selection_direction_applies() const;
     bool pattern_applies() const;
     bool multiple_applies() const;
+    bool required_applies() const;
     bool has_selectable_text() const;
 
     bool supports_a_picker() const;
diff --git a/Tests/LibWeb/Ref/expected/required-optional-pseudoclass-ref.html b/Tests/LibWeb/Ref/expected/required-optional-pseudoclass-ref.html
new file mode 100644
index 00000000000..2f4b65a907a
--- /dev/null
+++ b/Tests/LibWeb/Ref/expected/required-optional-pseudoclass-ref.html
@@ -0,0 +1,56 @@
+<!DOCTYPE html>
+
+<style>
+    input.required {
+        border: 2px solid red;
+    }
+
+    input.optional {
+        border: 2px solid blue;
+    }
+
+    input.required + span {
+        color: green;
+    }
+
+    div > input.required {
+        border: 2px dashed red;
+    }
+
+    section input.optional {
+        border: 2px dashed blue;
+    }
+
+    input.required ~ label {
+        color: orange;
+    }
+
+    form.has-required {
+        border: 2px solid purple;
+    }
+</style>
+
+<input type="text" class="required" placeholder="Should be red"><span>Should be green</span>
+<input type="text" class="optional" placeholder="Should be blue">
+
+<div>
+    <input type="text" class="required" placeholder="Should be dashed red">
+</div>
+
+<section>
+    <div>
+        <input type="text" class="optional" placeholder="Should be dashed blue">
+    </div>
+</section>
+
+<p>Other content</p>
+<label>Should be orange</label>
+
+<form class="has-required">
+    <span>Purple border</span>
+    <input id="make-me-required" class="required" type="text" placeholder="Should be red">
+</form>
+<form>
+    <span>No border</span>
+    <input id="make-me-optional" class="optional" type="text" placeholder="Should be blue">
+</form>
diff --git a/Tests/LibWeb/Ref/input/required-optional-pseudoclass.html b/Tests/LibWeb/Ref/input/required-optional-pseudoclass.html
new file mode 100644
index 00000000000..576ecdcb690
--- /dev/null
+++ b/Tests/LibWeb/Ref/input/required-optional-pseudoclass.html
@@ -0,0 +1,68 @@
+<!DOCTYPE html>
+<link rel="match" href="../expected/required-optional-pseudoclass-ref.html" />
+
+<style>
+    input:required {
+        border: 2px solid red;
+    }
+
+    input:optional {
+        border: 2px solid blue;
+    }
+
+    input:required+span {
+        color: green;
+    }
+
+    div>input:required {
+        border: 2px dashed red;
+    }
+
+    section input:optional {
+        border: 2px dashed blue;
+    }
+
+    input:required~label {
+        color: orange;
+    }
+
+    form:has(input:required) {
+        border: 2px solid purple;
+    }
+</style>
+
+<input type="text" required placeholder="Should be red"><span>Should be green</span>
+<input type="text" placeholder="Should be blue">
+
+<div>
+    <input type="text" required placeholder="Should be dashed red">
+</div>
+
+<section>
+    <div>
+        <input type="text" placeholder="Should be dashed blue">
+    </div>
+</section>
+
+<p>Other content</p>
+<label>Should be orange</label>
+
+<form>
+    <span>Purple border</span>
+    <input id="make-me-required" type="text" placeholder="Should be red">
+</form>
+<form>
+    <span>No border</span>
+    <input id="make-me-optional" type="text" required placeholder="Should be blue">
+</form>
+
+<!-- After the document has loaded, toggle the required-ness of the previous two inputs -->
+<script>
+    document.addEventListener("DOMContentLoaded", function () {
+        const makeMeOptional = document.getElementById("make-me-optional");
+        const makeMeRequired = document.getElementById("make-me-required");
+
+        makeMeOptional.required = false;
+        makeMeRequired.required = true;
+    });
+</script>
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-required-attribute.tentative.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-required-attribute.tentative.txt
index 5f23242531e..78d8b55c618 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-required-attribute.tentative.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/forms/the-select-element/select-required-attribute.tentative.txt
@@ -2,5 +2,5 @@ Harness status: OK
 
 Found 1 tests
 
-1 Fail
-Fail	Test required attribute
\ No newline at end of file
+1 Pass
+Pass	Test required attribute
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/required-optional-hidden.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/required-optional-hidden.txt
new file mode 100644
index 00000000000..7ae303f9feb
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/required-optional-hidden.txt
@@ -0,0 +1,6 @@
+Harness status: OK
+
+Found 1 tests
+
+1 Pass
+Pass	Evaluation of :required and :optional changes for input type change.
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/required-optional.txt b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/required-optional.txt
new file mode 100644
index 00000000000..26ea0f0969f
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/wpt-import/html/semantics/selectors/pseudo-classes/required-optional.txt
@@ -0,0 +1,11 @@
+Harness status: OK
+
+Found 6 tests
+
+6 Pass
+Pass	':required' matches required <input>s, <select>s and <textarea>s
+Pass	':optional' matches elements <input>s, <select>s and <textarea>s that are not required
+Pass	':required' doesn't match elements whose required attribute has been removed
+Pass	':optional' matches elements whose required attribute has been removed
+Pass	':required' matches elements whose required attribute has been added
+Pass	':optional' doesn't match elements whose required attribute has been added
\ No newline at end of file
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/required-optional-hidden.html b/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/required-optional-hidden.html
new file mode 100644
index 00000000000..6e8da0eaed8
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/required-optional-hidden.html
@@ -0,0 +1,36 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<title>Selector: pseudo-classes (:required, :optional) for hidden input</title>
+<link rel="author" title="Rune Lillesveen" href="mailto:rune@opera.com">
+<link rel="help" href="https://html.spec.whatwg.org/multipage/#pseudo-classes">
+<script src="../../../../resources/testharness.js"></script>
+<script src="../../../../resources/testharnessreport.js"></script>
+<style>
+  span {
+    color: red;
+    background-color: pink;
+  }
+  :required + span {
+    color: green;
+  }
+  :not(:optional) + span {
+    background-color: lime;
+  }
+</style>
+<input id="hiddenInput" type="hidden" required>
+<span id="sibling">This text should be green on lime background.</span>
+<script>
+  test(() => {
+    assert_equals(getComputedStyle(sibling).color, "rgb(255, 0, 0)",
+      "Not matching :required for type=hidden");
+    assert_equals(getComputedStyle(sibling).backgroundColor, "rgb(255, 192, 203)",
+      "Matching :optional for type=hidden");
+
+    hiddenInput.type = "text";
+
+    assert_equals(getComputedStyle(sibling).color, "rgb(0, 128, 0)",
+      "Matching :required for type=text");
+    assert_equals(getComputedStyle(sibling).backgroundColor, "rgb(0, 255, 0)",
+      "Matching :not(:optional) for type=text");
+  }, "Evaluation of :required and :optional changes for input type change.");
+</script>
diff --git a/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/required-optional.html b/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/required-optional.html
new file mode 100644
index 00000000000..1042856436f
--- /dev/null
+++ b/Tests/LibWeb/Text/input/wpt-import/html/semantics/selectors/pseudo-classes/required-optional.html
@@ -0,0 +1,35 @@
+<!DOCTYPE html>
+<meta charset=utf-8>
+<title>Selector: pseudo-classes (:required, :optional)</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>
+<input type=text id=text1 value="foobar" required>
+<input type=text id=text2 required>
+<input type=text id=text3>
+<select id=select1 required>
+ <optgroup label="options" id=optgroup1>
+   <option value="option1" id=option1>option1
+</select>
+<select id=select2>
+ <optgroup label="options" id=optgroup2>
+   <option value="option2" id=option2>option2
+</select>
+<textarea required id=textarea1>textarea1</textarea>
+<textarea id=textarea2>textarea2</textarea>
+
+<script>
+  testSelectorIdsMatch(":required", ["text1", "text2", "select1", "textarea1"], "':required' matches required <input>s, <select>s and <textarea>s");
+  testSelectorIdsMatch(":optional", ["text3", "select2", "textarea2"], "':optional' matches elements <input>s, <select>s and <textarea>s that are not required");
+
+  document.getElementById("text1").removeAttribute("required");
+  testSelectorIdsMatch(":required", ["text2", "select1", "textarea1"], "':required' doesn't match elements whose required attribute has been removed");
+  testSelectorIdsMatch(":optional", ["text1", "text3", "select2", "textarea2"], "':optional' matches elements whose required attribute has been removed");
+
+  document.getElementById("select2").setAttribute("required", "required");
+  testSelectorIdsMatch(":required", ["text2", "select1", "select2", "textarea1"], "':required' matches elements whose required attribute has been added");
+  testSelectorIdsMatch(":optional", ["text1", "text3", "textarea2"], "':optional' doesn't match elements whose required attribute has been added");
+</script>