diff --git a/Userland/Libraries/LibWeb/DOM/Document.idl b/Userland/Libraries/LibWeb/DOM/Document.idl
index dcc07f2590c..1516b01b4ea 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.idl
+++ b/Userland/Libraries/LibWeb/DOM/Document.idl
@@ -71,8 +71,8 @@ interface Document : Node {
 
     Element? getElementById(DOMString id);
     HTMLCollection getElementsByName([FlyString] DOMString name);
-    HTMLCollection getElementsByTagName(DOMString tagName);
-    HTMLCollection getElementsByTagNameNS(DOMString? namespace, DOMString localName);
+    HTMLCollection getElementsByTagName([FlyString] DOMString tagName);
+    HTMLCollection getElementsByTagNameNS([FlyString] DOMString? namespace, [FlyString] DOMString localName);
     HTMLCollection getElementsByClassName(DOMString className);
 
     [SameObject] readonly attribute HTMLCollection applets;
diff --git a/Userland/Libraries/LibWeb/DOM/Element.idl b/Userland/Libraries/LibWeb/DOM/Element.idl
index fe96e6ad14e..e0484481f41 100644
--- a/Userland/Libraries/LibWeb/DOM/Element.idl
+++ b/Userland/Libraries/LibWeb/DOM/Element.idl
@@ -60,8 +60,8 @@ interface Element : Node {
     Element? closest(DOMString selectors);
     [ImplementedAs=matches] boolean webkitMatchesSelector(DOMString selectors); // legacy alias of .matches
 
-    HTMLCollection getElementsByTagName(DOMString tagName);
-    HTMLCollection getElementsByTagNameNS(DOMString? namespace, DOMString localName);
+    HTMLCollection getElementsByTagName([FlyString] DOMString tagName);
+    HTMLCollection getElementsByTagNameNS([FlyString] DOMString? namespace, [FlyString] DOMString localName);
     HTMLCollection getElementsByClassName(DOMString className);
 
     [CEReactions] Element? insertAdjacentElement(DOMString where, Element element); // legacy
diff --git a/Userland/Libraries/LibWeb/DOM/ParentNode.cpp b/Userland/Libraries/LibWeb/DOM/ParentNode.cpp
index 59c5a85dfdd..f32a8b7d271 100644
--- a/Userland/Libraries/LibWeb/DOM/ParentNode.cpp
+++ b/Userland/Libraries/LibWeb/DOM/ParentNode.cpp
@@ -156,12 +156,11 @@ JS::NonnullGCPtr<HTMLCollection> ParentNode::get_elements_by_tag_name(FlyString
 
 // https://dom.spec.whatwg.org/#concept-getelementsbytagnamens
 // NOTE: This method is only exposed on Document and Element, but is in ParentNode to prevent code duplication.
-JS::NonnullGCPtr<HTMLCollection> ParentNode::get_elements_by_tag_name_ns(Optional<String> const& nullable_namespace, FlyString const& local_name)
+JS::NonnullGCPtr<HTMLCollection> ParentNode::get_elements_by_tag_name_ns(Optional<FlyString> namespace_, FlyString const& local_name)
 {
     // 1. If namespace is the empty string, set it to null.
-    Optional<FlyString> namespace_;
-    if (nullable_namespace.has_value() && !nullable_namespace->is_empty())
-        namespace_ = nullable_namespace.value();
+    if (namespace_ == FlyString {})
+        namespace_ = OptionalNone {};
 
     // 2. If both namespace and localName are "*" (U+002A), return a HTMLCollection rooted at root, whose filter matches descendant elements.
     if (namespace_ == "*" && local_name == "*") {
diff --git a/Userland/Libraries/LibWeb/DOM/ParentNode.h b/Userland/Libraries/LibWeb/DOM/ParentNode.h
index a8a56571dc5..184f1d93116 100644
--- a/Userland/Libraries/LibWeb/DOM/ParentNode.h
+++ b/Userland/Libraries/LibWeb/DOM/ParentNode.h
@@ -30,8 +30,7 @@ public:
     JS::NonnullGCPtr<HTMLCollection> children();
 
     JS::NonnullGCPtr<HTMLCollection> get_elements_by_tag_name(FlyString const&);
-    // FIXME: This should take an Optional<FlyString>
-    JS::NonnullGCPtr<HTMLCollection> get_elements_by_tag_name_ns(Optional<String> const&, FlyString const&);
+    JS::NonnullGCPtr<HTMLCollection> get_elements_by_tag_name_ns(Optional<FlyString>, FlyString const&);
 
     WebIDL::ExceptionOr<void> prepend(Vector<Variant<JS::Handle<Node>, String>> const& nodes);
     WebIDL::ExceptionOr<void> append(Vector<Variant<JS::Handle<Node>, String>> const& nodes);