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chromium/chrome/test/data/extensions/platform_apps/web_view/load_webview_inside_iframe/embedder.html 

<!doctype html>
<!--
Copyright 2017 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>
  <!--
   * Do not include any scripts in this file as that would create a script
   * context for the iframe. We want to test that the browser ensures the
   * creation of a script context so that this webview is recognized as a
   * custom element.
  -->
  <webview></webview>
</body>
</html>