diff --git a/lint.ignore b/lint.ignore
index 8d0ac45b8c88ab..23cf8609bf2540 100644
--- a/lint.ignore
+++ b/lint.ignore
@@ -688,6 +688,7 @@ MISSING-LINK: css/filter-effects/*.any.js
 
 # Tests that use WebKit/Blink testing APIs
 LAYOUTTESTS APIS: import-maps/common/resources/common-test-helper.js
+LAYOUTTESTS APIS: resources/test-only-api.js
 LAYOUTTESTS APIS: resources/chromium/enable-hyperlink-auditing.js
 LAYOUTTESTS APIS: resources/chromium/generic_sensor_mocks.js
 LAYOUTTESTS APIS: resources/chromium/nfc-mock.js
@@ -709,6 +710,7 @@ WEB-PLATFORM.TEST:web-bundle/resources/wbn/*.wbn
 # Tests that depend on resources in /gen/ in Chromium:
 # https://github.com/web-platform-tests/wpt/issues/16455
 # Please consult with ecosystem-infra@chromium.org before adding more.
+MISSING DEPENDENCY: resources/test-only-api.js
 MISSING DEPENDENCY: idle-detection/interceptor.https.html
 MISSING DEPENDENCY: credential-management/support/otpcredential-helper.js
 MISSING DEPENDENCY: web-nfc/resources/nfc-helpers.js
diff --git a/resources/test-only-api.js b/resources/test-only-api.js
new file mode 100644
index 00000000000000..1247e295360e4d
--- /dev/null
+++ b/resources/test-only-api.js
@@ -0,0 +1,68 @@
+'use strict';
+
+/* Whether the browser is Chromium-based with MojoJS enabled */
+const isChromiumBased = 'MojoInterfaceInterceptor' in self;
+/* Whether the browser is WebKit-based with internal test-only API enabled */
+const isWebKitBased = !isChromiumBased && 'internals' in self;
+
+/**
+ * Loads a script in a window or worker.
+ *
+ * @param {string} path - A script path
+ * @returns {Promise}
+ */
+function loadScript(path) {
+  if (typeof document === 'undefined') {
+    // Workers (importScripts is synchronous and may throw.)
+    importScripts(path);
+    return Promise.resolve();
+  } else {
+    // Window
+    const script = document.createElement('script');
+    script.src = path;
+    script.async = false;
+    const p = new Promise((resolve, reject) => {
+      script.onload = () => { resolve(); };
+      script.onerror = e => { reject(e); };
+    })
+    document.head.appendChild(script);
+    return p;
+  }
+}
+
+/**
+ * A helper for Chromium-based browsers to load Mojo JS bindingds
+ *
+ * This is an async function that works in both workers and windows. It first
+ * loads mojo_bindings.js, disables automatic dependency loading, and loads all
+ * resources sequentially. The promise resolves if everything loads
+ * successfully, or rejects if any exception is raised. If testharness.js is
+ * used, an uncaught exception will terminate the test with a harness error
+ * (unless `allow_uncaught_exception` is true), which is usually the desired
+ * behaviour. Only call this function if isChromiumBased === true.
+ *
+ * @param {Array.<string>} resources - A list of scripts to load: Mojo JS
+ *   bindings should be of the form '/gen/../*.mojom.js', the ordering of which
+ *   does not matter. Do not include mojo_bindings.js in this list. You may
+ *   include other non-mojom.js scripts for convenience.
+ * @returns {Promise}
+ */
+async function loadMojoResources(resources) {
+  if (!('MojoInterfaceInterceptor' in self)) {
+    throw new Error('MojoJS not enabled; start Chrome with --enable-blink-features=MojoJS,MojoJSTest');
+  }
+  if (resources.length == 0) {
+    return;
+  }
+
+  // We want to load mojo_bindings.js separately to set mojo.config.
+  if (resources.some(p => p.endsWith('/mojo_bindings.js'))) {
+    throw new Error('Do not load mojo_bindings.js explicitly.');
+  }
+  await loadScript('/gen/layout_test_data/mojo/public/js/mojo_bindings.js');
+  mojo.config.autoLoadMojomDeps = false;
+
+  for (const path of resources) {
+    await loadScript(path);
+  }
+}
diff --git a/resources/test-only-api.js.headers b/resources/test-only-api.js.headers
new file mode 100644
index 00000000000000..5e8f640c6659d1
--- /dev/null
+++ b/resources/test-only-api.js.headers
@@ -0,0 +1,2 @@
+Content-Type: text/javascript; charset=utf-8
+Cache-Control: max-age=3600
diff --git a/webxr/resources/webxr_util.js b/webxr/resources/webxr_util.js
index 81dc3d620aa1f7..a987ca1f4420c1 100644
--- a/webxr/resources/webxr_util.js
+++ b/webxr/resources/webxr_util.js
@@ -1,3 +1,5 @@
+'use strict';
+
 // These tests rely on the User Agent providing an implementation of the
 // WebXR Testing API (https://github.com/immersive-web/webxr-test-api).
 //
@@ -9,22 +11,34 @@
 
 // Debugging message helper, by default does nothing. Implementations can
 // override this.
-var xr_debug = function(name, msg) {}
-var isChromiumBased = 'MojoInterfaceInterceptor' in self;
-var isWebKitBased = 'internals' in self && 'xrTest' in internals;
+var xr_debug = function(name, msg) {};
 
 function xr_promise_test(name, func, properties) {
   promise_test(async (t) => {
     // Perform any required test setup:
     xr_debug(name, 'setup');
 
+    // Only set up once.
     if (!navigator.xr.test) {
+      // Load test-only API helpers.
+      const script = document.createElement('script');
+      script.src = '/resources/test-only-api.js';
+      script.async = false;
+      const p = new Promise((resolve, reject) => {
+        script.onload = () => { resolve(); };
+        script.onerror = e => { reject(e); };
+      })
+      document.head.appendChild(script);
+      await p;
+
       if (isChromiumBased) {
         // Chrome setup
         await loadChromiumResources();
       } else if (isWebKitBased) {
         // WebKit setup
         await setupWebKitWebXRTestAPI();
+      } else {
+        assert_implements(false, "missing navigator.xr.test");
       }
     }
 
@@ -185,19 +199,27 @@ function forEachWebxrObject(callback) {
 }
 
 // Code for loading test API in Chromium.
-function loadChromiumResources() {
+async function loadChromiumResources() {
   let chromiumResources = [
-    '/gen/layout_test_data/mojo/public/js/mojo_bindings.js',
     '/gen/mojo/public/mojom/base/time.mojom.js',
-    '/gen/gpu/ipc/common/mailbox_holder.mojom.js',
+    '/gen/mojo/public/mojom/base/shared_memory.mojom.js',
+    '/gen/mojo/public/mojom/base/unguessable_token.mojom.js',
     '/gen/gpu/ipc/common/sync_token.mojom.js',
-    '/gen/ui/display/mojom/display.mojom.js',
+    '/gen/gpu/ipc/common/mailbox.mojom.js',
+    '/gen/gpu/ipc/common/mailbox_holder.mojom.js',
     '/gen/ui/gfx/geometry/mojom/geometry.mojom.js',
+    '/gen/ui/gfx/mojom/native_handle_types.mojom.js',
+    '/gen/ui/gfx/mojom/buffer_types.mojom.js',
+    '/gen/ui/gfx/mojom/color_space.mojom.js',
+    '/gen/ui/gfx/mojom/display_color_spaces.mojom.js',
     '/gen/ui/gfx/mojom/gpu_fence_handle.mojom.js',
     '/gen/ui/gfx/mojom/transform.mojom.js',
+    '/gen/ui/display/mojom/display.mojom.js',
+    '/gen/device/gamepad/public/mojom/gamepad.mojom.js',
     '/gen/device/vr/public/mojom/vr_service.mojom.js',
     '/resources/chromium/webxr-test-math-helper.js',
     '/resources/chromium/webxr-test.js',
+    // Required only by resources/chromium/webxr-test.js
     '/resources/testdriver.js',
     '/resources/testdriver-vendor.js',
   ];
@@ -210,22 +232,9 @@ function loadChromiumResources() {
     chromiumResources = chromiumResources.concat(additionalChromiumResources);
   }
 
-  let chain = Promise.resolve();
-    chromiumResources.forEach(path => {
-      let script = document.createElement('script');
-      script.src = path;
-      script.async = false;
-      chain = chain.then(() => new Promise(resolve => {
-                           script.onload = () => resolve();
-                         }));
-      document.head.appendChild(script);
-  });
-
-  chain = chain.then(() => {
-    xr_debug = navigator.xr.test.Debug;
-  });
+  await loadMojoResources(chromiumResources);
 
-  return chain;
+  xr_debug = navigator.xr.test.Debug;
 }
 
 function setupWebKitWebXRTestAPI() {