<?php
header('Cache-Control: s-max-age=1, stale-while-revalidate=99999999');
sleep(5);
print('This is a slow test with as fresh data as possible. ' . date('Y-m-d H:i:s'));
?>
<p id="js-date"></p>
<script type="text/javascript">
  document.getElementById('js-date').innerText = new Date().toISOString();
</script>