diff --git a/Libraries/LibWeb/DOM/ParentNode.cpp b/Libraries/LibWeb/DOM/ParentNode.cpp
index 1932c78c9b8..d8f918f8665 100644
--- a/Libraries/LibWeb/DOM/ParentNode.cpp
+++ b/Libraries/LibWeb/DOM/ParentNode.cpp
@@ -40,6 +40,33 @@ static WebIDL::ExceptionOr<Variant<GC::Ptr<Element>, GC::Ref<NodeList>>> scope_m
 
     auto selectors = maybe_selectors.value();
 
+    bool has_named_namespace = false;
+    for (auto const& selector : selectors) {
+        for (auto const& compound_selector : selector->compound_selectors()) {
+            for (auto simple_selector : compound_selector.simple_selectors) {
+                if (simple_selector.value.has<CSS::Selector::SimpleSelector::QualifiedName>() && simple_selector.qualified_name().namespace_type == CSS::Selector::SimpleSelector::QualifiedName::NamespaceType::Named) {
+                    has_named_namespace = true;
+                    break;
+                }
+
+                if (simple_selector.value.has<CSS::Selector::SimpleSelector::PseudoClassSelector>()) {
+                    for (auto const& argument_selector : simple_selector.pseudo_class().argument_selector_list) {
+                        for (auto const& argument_compound_selector : argument_selector->compound_selectors()) {
+                            for (auto argument_simple_selector : argument_compound_selector.simple_selectors) {
+                                if (argument_simple_selector.value.has<CSS::Selector::SimpleSelector::QualifiedName>() && argument_simple_selector.qualified_name().namespace_type == CSS::Selector::SimpleSelector::QualifiedName::NamespaceType::Named) {
+                                    has_named_namespace = true;
+                                    break;
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+    if (has_named_namespace)
+        return WebIDL::SyntaxError::create(node.realm(), "Failed to parse selector"_string);
+
     // 3. Return the result of match a selector against a tree with s and node’s root using scoping root node.
     GC::Ptr<Element> single_result;
     Vector<GC::Root<Node>> results;
diff --git a/Tests/LibWeb/Text/expected/wpt-import/dom/nodes/ParentNode-querySelector-All.txt b/Tests/LibWeb/Text/expected/wpt-import/dom/nodes/ParentNode-querySelector-All.txt
index 29487d7992a..34ace2711f5 100644
--- a/Tests/LibWeb/Text/expected/wpt-import/dom/nodes/ParentNode-querySelector-All.txt
+++ b/Tests/LibWeb/Text/expected/wpt-import/dom/nodes/ParentNode-querySelector-All.txt
@@ -6,8 +6,8 @@ Rerun
 
 Found 1975 tests
 
-1931 Pass
-44 Fail
+1951 Pass
+24 Fail
 Details
 Result	Test Name	MessagePass	Selectors-API Test Suite: HTML	
 Pass	Document supports querySelector	
@@ -116,10 +116,10 @@ Pass	Document.querySelector: Invalid pseudo-element: :::before
 Pass	Document.querySelectorAll: Invalid pseudo-element: :::before	
 Pass	Document.querySelector: Invalid pseudo-element: :: before	
 Pass	Document.querySelectorAll: Invalid pseudo-element: :: before	
-Fail	Document.querySelector: Undeclared namespace: ns|div	
-Fail	Document.querySelectorAll: Undeclared namespace: ns|div	
-Fail	Document.querySelector: Undeclared namespace: :not(ns|div)	
-Fail	Document.querySelectorAll: Undeclared namespace: :not(ns|div)	
+Pass	Document.querySelector: Undeclared namespace: ns|div	
+Pass	Document.querySelectorAll: Undeclared namespace: ns|div	
+Pass	Document.querySelector: Undeclared namespace: :not(ns|div)	
+Pass	Document.querySelectorAll: Undeclared namespace: :not(ns|div)	
 Pass	Document.querySelector: Invalid namespace: ^|div	
 Pass	Document.querySelectorAll: Invalid namespace: ^|div	
 Pass	Document.querySelector: Invalid namespace: $|div	
@@ -184,10 +184,10 @@ Pass	Detached Element.querySelector: Invalid pseudo-element: :::before
 Pass	Detached Element.querySelectorAll: Invalid pseudo-element: :::before	
 Pass	Detached Element.querySelector: Invalid pseudo-element: :: before	
 Pass	Detached Element.querySelectorAll: Invalid pseudo-element: :: before	
-Fail	Detached Element.querySelector: Undeclared namespace: ns|div	
-Fail	Detached Element.querySelectorAll: Undeclared namespace: ns|div	
-Fail	Detached Element.querySelector: Undeclared namespace: :not(ns|div)	
-Fail	Detached Element.querySelectorAll: Undeclared namespace: :not(ns|div)	
+Pass	Detached Element.querySelector: Undeclared namespace: ns|div	
+Pass	Detached Element.querySelectorAll: Undeclared namespace: ns|div	
+Pass	Detached Element.querySelector: Undeclared namespace: :not(ns|div)	
+Pass	Detached Element.querySelectorAll: Undeclared namespace: :not(ns|div)	
 Pass	Detached Element.querySelector: Invalid namespace: ^|div	
 Pass	Detached Element.querySelectorAll: Invalid namespace: ^|div	
 Pass	Detached Element.querySelector: Invalid namespace: $|div	
@@ -252,10 +252,10 @@ Pass	Fragment.querySelector: Invalid pseudo-element: :::before
 Pass	Fragment.querySelectorAll: Invalid pseudo-element: :::before	
 Pass	Fragment.querySelector: Invalid pseudo-element: :: before	
 Pass	Fragment.querySelectorAll: Invalid pseudo-element: :: before	
-Fail	Fragment.querySelector: Undeclared namespace: ns|div	
-Fail	Fragment.querySelectorAll: Undeclared namespace: ns|div	
-Fail	Fragment.querySelector: Undeclared namespace: :not(ns|div)	
-Fail	Fragment.querySelectorAll: Undeclared namespace: :not(ns|div)	
+Pass	Fragment.querySelector: Undeclared namespace: ns|div	
+Pass	Fragment.querySelectorAll: Undeclared namespace: ns|div	
+Pass	Fragment.querySelector: Undeclared namespace: :not(ns|div)	
+Pass	Fragment.querySelectorAll: Undeclared namespace: :not(ns|div)	
 Pass	Fragment.querySelector: Invalid namespace: ^|div	
 Pass	Fragment.querySelectorAll: Invalid namespace: ^|div	
 Pass	Fragment.querySelector: Invalid namespace: $|div	
@@ -320,10 +320,10 @@ Pass	In-document Element.querySelector: Invalid pseudo-element: :::before
 Pass	In-document Element.querySelectorAll: Invalid pseudo-element: :::before	
 Pass	In-document Element.querySelector: Invalid pseudo-element: :: before	
 Pass	In-document Element.querySelectorAll: Invalid pseudo-element: :: before	
-Fail	In-document Element.querySelector: Undeclared namespace: ns|div	
-Fail	In-document Element.querySelectorAll: Undeclared namespace: ns|div	
-Fail	In-document Element.querySelector: Undeclared namespace: :not(ns|div)	
-Fail	In-document Element.querySelectorAll: Undeclared namespace: :not(ns|div)	
+Pass	In-document Element.querySelector: Undeclared namespace: ns|div	
+Pass	In-document Element.querySelectorAll: Undeclared namespace: ns|div	
+Pass	In-document Element.querySelector: Undeclared namespace: :not(ns|div)	
+Pass	In-document Element.querySelectorAll: Undeclared namespace: :not(ns|div)	
 Pass	In-document Element.querySelector: Invalid namespace: ^|div	
 Pass	In-document Element.querySelectorAll: Invalid namespace: ^|div	
 Pass	In-document Element.querySelector: Invalid namespace: $|div	
@@ -388,10 +388,10 @@ Pass	Empty Element.querySelector: Invalid pseudo-element: :::before
 Pass	Empty Element.querySelectorAll: Invalid pseudo-element: :::before	
 Pass	Empty Element.querySelector: Invalid pseudo-element: :: before	
 Pass	Empty Element.querySelectorAll: Invalid pseudo-element: :: before	
-Fail	Empty Element.querySelector: Undeclared namespace: ns|div	
-Fail	Empty Element.querySelectorAll: Undeclared namespace: ns|div	
-Fail	Empty Element.querySelector: Undeclared namespace: :not(ns|div)	
-Fail	Empty Element.querySelectorAll: Undeclared namespace: :not(ns|div)	
+Pass	Empty Element.querySelector: Undeclared namespace: ns|div	
+Pass	Empty Element.querySelectorAll: Undeclared namespace: ns|div	
+Pass	Empty Element.querySelector: Undeclared namespace: :not(ns|div)	
+Pass	Empty Element.querySelectorAll: Undeclared namespace: :not(ns|div)	
 Pass	Empty Element.querySelector: Invalid namespace: ^|div	
 Pass	Empty Element.querySelectorAll: Invalid namespace: ^|div	
 Pass	Empty Element.querySelector: Invalid namespace: $|div