diff --git a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
index 76716bf3d1..59d1a7f173 100644
--- a/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
+++ b/Userland/Libraries/LibWeb/CSS/Parser/Parser.cpp
@@ -447,6 +447,8 @@ Parser::ParseErrorOr<Selector::SimpleSelector> Parser::parse_pseudo_simple_selec
             return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Active);
         if (pseudo_name.equals_ignoring_ascii_case("checked"sv))
             return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Checked);
+        if (pseudo_name.equals_ignoring_ascii_case("indeterminate"sv))
+            return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Indeterminate);
         if (pseudo_name.equals_ignoring_ascii_case("disabled"sv))
             return make_pseudo_class_selector(Selector::SimpleSelector::PseudoClass::Type::Disabled);
         if (pseudo_name.equals_ignoring_ascii_case("empty"sv))
diff --git a/Userland/Libraries/LibWeb/CSS/Selector.h b/Userland/Libraries/LibWeb/CSS/Selector.h
index 87d269f54a..dd1da6cdbd 100644
--- a/Userland/Libraries/LibWeb/CSS/Selector.h
+++ b/Userland/Libraries/LibWeb/CSS/Selector.h
@@ -105,6 +105,7 @@ public:
                 Disabled,
                 Enabled,
                 Checked,
+                Indeterminate,
                 Is,
                 Not,
                 Where,
@@ -275,6 +276,8 @@ constexpr StringView pseudo_class_name(Selector::SimpleSelector::PseudoClass::Ty
         return "enabled"sv;
     case Selector::SimpleSelector::PseudoClass::Type::Checked:
         return "checked"sv;
+    case Selector::SimpleSelector::PseudoClass::Type::Indeterminate:
+        return "indeterminate"sv;
     case Selector::SimpleSelector::PseudoClass::Type::Active:
         return "active"sv;
     case Selector::SimpleSelector::PseudoClass::Type::NthChild:
diff --git a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
index 7ad6d480be..92e8c1313f 100644
--- a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
+++ b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
@@ -19,6 +19,7 @@
 #include <LibWeb/HTML/HTMLInputElement.h>
 #include <LibWeb/HTML/HTMLOptGroupElement.h>
 #include <LibWeb/HTML/HTMLOptionElement.h>
+#include <LibWeb/HTML/HTMLProgressElement.h>
 #include <LibWeb/HTML/HTMLSelectElement.h>
 #include <LibWeb/HTML/HTMLTextAreaElement.h>
 #include <LibWeb/Infra/Strings.h>
@@ -96,6 +97,28 @@ static inline bool matches_checked_pseudo_class(DOM::Element const& element)
     return false;
 }
 
+// https://html.spec.whatwg.org/multipage/semantics-other.html#selector-indeterminate
+static inline bool matches_indeterminate_pseudo_class(DOM::Element const& element)
+{
+    // The :indeterminate pseudo-class must match any element falling into one of the following categories:
+    // - input elements whose type attribute is in the Checkbox state and whose indeterminate IDL attribute is set to true
+    // FIXME: - input elements whose type attribute is in the Radio Button state and whose radio button group contains no input elements whose checkedness state is true.
+    if (is<HTML::HTMLInputElement>(element)) {
+        auto const& input_element = static_cast<HTML::HTMLInputElement const&>(element);
+        switch (input_element.type_state()) {
+        case HTML::HTMLInputElement::TypeAttributeState::Checkbox:
+            return input_element.indeterminate();
+        default:
+            return false;
+        }
+    }
+    // - progress elements with no value content attribute
+    if (is<HTML::HTMLProgressElement>(element)) {
+        return !element.has_attribute(HTML::AttributeNames::value);
+    }
+    return false;
+}
+
 static inline bool matches_attribute(CSS::Selector::SimpleSelector::Attribute const& attribute, DOM::Element const& element)
 {
     if (attribute.match_type == CSS::Selector::SimpleSelector::Attribute::MatchType::HasAttribute) {
@@ -241,6 +264,8 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
             && !element.is_actually_disabled();
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Checked:
         return matches_checked_pseudo_class(element);
+    case CSS::Selector::SimpleSelector::PseudoClass::Type::Indeterminate:
+        return matches_indeterminate_pseudo_class(element);
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Is:
     case CSS::Selector::SimpleSelector::PseudoClass::Type::Where:
         for (auto& selector : pseudo_class.argument_selector_list) {
diff --git a/Userland/Libraries/LibWeb/Dump.cpp b/Userland/Libraries/LibWeb/Dump.cpp
index ee6de8ae3b..666924b989 100644
--- a/Userland/Libraries/LibWeb/Dump.cpp
+++ b/Userland/Libraries/LibWeb/Dump.cpp
@@ -441,6 +441,9 @@ void dump_selector(StringBuilder& builder, CSS::Selector const& selector)
                 case CSS::Selector::SimpleSelector::PseudoClass::Type::Checked:
                     pseudo_class_description = "Checked";
                     break;
+                case CSS::Selector::SimpleSelector::PseudoClass::Type::Indeterminate:
+                    pseudo_class_description = "Indeterminate";
+                    break;
                 case CSS::Selector::SimpleSelector::PseudoClass::Type::Not:
                     pseudo_class_description = "Not";
                     break;