diff --git a/components/script/dom/webidls/XRTest.webidl b/components/script/dom/webidls/XRTest.webidl
index 1b1d131d562..89af1da991e 100644
--- a/components/script/dom/webidls/XRTest.webidl
+++ b/components/script/dom/webidls/XRTest.webidl
@@ -13,7 +13,7 @@ interface XRTest {
   // // Simulates a user activation (aka user gesture) for the current scope.
   // // The activation is only guaranteed to be valid in the provided function and only applies to WebXR
   // // Device API methods.
-  // void simulateUserActivation(Function);
+  void simulateUserActivation(Function f);
 
   // // Disconnect all fake devices
   // Promise<void> disconnectAllDevices();
diff --git a/components/script/dom/xrtest.rs b/components/script/dom/xrtest.rs
index 6068a9a3e3a..8049b9ffa41 100644
--- a/components/script/dom/xrtest.rs
+++ b/components/script/dom/xrtest.rs
@@ -6,6 +6,8 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use crate::dom::bindings::callback::ExceptionHandling;
+use crate::dom::bindings::codegen::Bindings::FunctionBinding::Function;
 use crate::dom::bindings::codegen::Bindings::XRTestBinding::{
     self, FakeXRDeviceInit, XRTestMethods,
 };
@@ -162,4 +164,10 @@ impl XRTestMethods for XRTest {
 
         p
     }
+
+    /// https://github.com/immersive-web/webxr-test-api/blob/master/explainer.md
+    fn SimulateUserActivation(&self, f: Rc<Function>) {
+        // XXXManishearth actually check for activation in XRSession
+        let _ = f.Call__(vec![], ExceptionHandling::Rethrow);
+    }
 }