Add test for the unsupported browser screen

playwright/e2e/app-loading/feature-detection.spec.js

@@ -0,0 +1,25 @@
+/*
+Copyright 2024 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { expect, test } from "@playwright/test";
+
+test(`shows error page if browser lacks Intl support`, async ({ page }) => {
+    await page.addInitScript({ content: `delete window.Intl;` });
+    await page.goto("/");
+
+    const header = await page.frameLocator("iframe").getByText("Unsupported browser");
+    await expect(header).toBeVisible();
+});