chromium/chrome/test/data/extensions/platform_apps/web_view/newwindow/subframe_with_webview.html

<!doctype html>
<!--
Copyright 2021 The Chromium Authors
Use of this source code is governed by a BSD-style license that can be
found in the LICENSE file.
-->
<html>
<body>
  <div>This page will be a sub-frame that embeds a webview.</div>
  <webview></webview>
</body>
</html>