diff --git a/components/script/dom/mouseevent.rs b/components/script/dom/mouseevent.rs
index aff9c4118f7..fe88f0b3356 100644
--- a/components/script/dom/mouseevent.rs
+++ b/components/script/dom/mouseevent.rs
@@ -527,4 +527,23 @@ impl MouseEventMethods<crate::DomTypeHolder> for MouseEvent {
     fn IsTrusted(&self) -> bool {
         self.uievent.IsTrusted()
     }
+
+    /// <https://w3c.github.io/uievents/#dom-mouseevent-getmodifierstate>
+    fn GetModifierState(&self, key_arg: DOMString) -> bool {
+        self.modifiers.get().contains(match &*key_arg {
+            "Alt" => Modifiers::ALT,
+            "AltGraph" => Modifiers::ALT_GRAPH,
+            "CapsLock" => Modifiers::CAPS_LOCK,
+            "Control" => Modifiers::CONTROL,
+            "Fn" => Modifiers::FN,
+            "FnLock" => Modifiers::FN_LOCK,
+            "Meta" => Modifiers::META,
+            "NumLock" => Modifiers::NUM_LOCK,
+            "ScrollLock" => Modifiers::SCROLL_LOCK,
+            "Shift" => Modifiers::SHIFT,
+            "Symbol" => Modifiers::SYMBOL,
+            "SymbolLock" => Modifiers::SYMBOL_LOCK,
+            _ => return false,
+        })
+    }
 }
diff --git a/components/script_bindings/webidls/MouseEvent.webidl b/components/script_bindings/webidls/MouseEvent.webidl
index 0a3c5c015fd..9726ea14506 100644
--- a/components/script_bindings/webidls/MouseEvent.webidl
+++ b/components/script_bindings/webidls/MouseEvent.webidl
@@ -24,7 +24,7 @@ interface MouseEvent : UIEvent {
     readonly    attribute EventTarget?   relatedTarget;
     // Introduced in DOM Level 3
     readonly    attribute unsigned short buttons;
-    //boolean getModifierState (DOMString keyArg);
+    boolean getModifierState (DOMString keyArg);
 
     [Pref="dom_mouse_event_which_enabled"]
     readonly    attribute long           which;
diff --git a/tests/wpt/meta/uievents/idlharness.window.js.ini b/tests/wpt/meta/uievents/idlharness.window.js.ini
index 55e71ccaf64..4a777dd66a7 100644
--- a/tests/wpt/meta/uievents/idlharness.window.js.ini
+++ b/tests/wpt/meta/uievents/idlharness.window.js.ini
@@ -11,24 +11,9 @@
   [UIEvent interface: new FocusEvent("event") must inherit property "which" with the proper type]
     expected: FAIL
 
-  [MouseEvent interface: operation getModifierState(DOMString)]
-    expected: FAIL
-
   [MouseEvent interface: operation initMouseEvent(DOMString, optional boolean, optional boolean, optional Window?, optional long, optional long, optional long, optional long, optional long, optional boolean, optional boolean, optional boolean, optional boolean, optional short, optional EventTarget?)]
     expected: FAIL
 
-  [MouseEvent interface: new MouseEvent("event") must inherit property "getModifierState(DOMString)" with the proper type]
-    expected: FAIL
-
-  [MouseEvent interface: calling getModifierState(DOMString) on new MouseEvent("event") with too few arguments must throw TypeError]
-    expected: FAIL
-
-  [MouseEvent interface: new WheelEvent("event") must inherit property "getModifierState(DOMString)" with the proper type]
-    expected: FAIL
-
-  [MouseEvent interface: calling getModifierState(DOMString) on new WheelEvent("event") with too few arguments must throw TypeError]
-    expected: FAIL
-
   [InputEvent interface: attribute inputType]
     expected: FAIL