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chromium/third_party/blink/web_tests/external/wpt/web-animations/interfaces/Document/timeline.html 

<!doctype html>
<meta charset=utf-8>
<title>Document.timeline</title>
<link rel="help" href="https://drafts.csswg.org/web-animations/#dom-document-timeline">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="../../testcommon.js"></script>
<div id="log"></div>
<iframe width="10" height="10" id="iframe"></iframe>
<script>
'use strict';

test(() => {
  assert_equals(document.timeline, document.timeline,
    'Document.timeline returns the same object every time');
  const iframe = document.getElementById('iframe');
  assert_not_equals(document.timeline, iframe.contentDocument.timeline,
    'Document.timeline returns a different object for each document');
  assert_not_equals(iframe.contentDocument.timeline, null,
    'Document.timeline on an iframe is not null');
}, 'Document.timeline returns the default document timeline');

</script>