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chromium/ash/webui/sample_system_web_app_ui/resources/trusted/timer.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. -->
<!DOCTYPE html>
<meta charset="utf-8">
<title>Sample System Web App timer page</title>
<header id='title'>Timer Page</header>

<p>
  This is a blank background page used to open the shared worker.
</p>
<script src="timer.js"></script>