Codebase Browser
chromium
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chromium/third_party/blink/web_tests/external/wpt/page-visibility/resources/iframe.html 

<!doctype html>
<html>
<head><title>Document</title></head>
<body>
<h1>Document</h1>
<script>
onload = function() {
  parent.startTest();
}
</script>
</body>
</html>