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chromium/chrome/test/data/downloads/download-anchor-attrib-400.html 

<html>
<head><title>&lt;a download&gt; that results in a status code of 400</title></head>
<body>
<a id='download' href="zip_file_not_found.zip" download='foo'>Download</a>
<script>
window.setTimeout(function() {
  var evt = document.createEvent("MouseEvent");
  evt.initMouseEvent('click', true, true);
  document.getElementById('download').dispatchEvent(evt);
}, 0);
</script>
</body>
</html>