diff --git a/Userland/Libraries/LibWeb/CSS/PseudoClasses.json b/Userland/Libraries/LibWeb/CSS/PseudoClasses.json
index 25f68d921d..c74a23fb2e 100644
--- a/Userland/Libraries/LibWeb/CSS/PseudoClasses.json
+++ b/Userland/Libraries/LibWeb/CSS/PseudoClasses.json
@@ -113,6 +113,9 @@
   "target": {
     "argument": ""
   },
+  "target-within": {
+    "argument": ""
+  },
   "visited": {
     "argument": ""
   },
diff --git a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
index e7a25e9d7a..1bbf994edc 100644
--- a/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
+++ b/Userland/Libraries/LibWeb/CSS/SelectorEngine.cpp
@@ -446,6 +446,12 @@ static inline bool matches_pseudo_class(CSS::Selector::SimpleSelector::PseudoCla
     }
     case CSS::PseudoClass::Target:
         return element.is_target();
+    case CSS::PseudoClass::TargetWithin: {
+        auto* target_element = element.document().target_element();
+        if (!target_element)
+            return false;
+        return element.is_inclusive_ancestor_of(*target_element);
+    }
     case CSS::PseudoClass::Dir: {
         // "Values other than ltr and rtl are not invalid, but do not match anything."
         // - https://www.w3.org/TR/selectors-4/#the-dir-pseudo