HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>doctype vs compatMode</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var info = {
      hasDoctype: document.doctype !== null,
      name: document.doctype ? document.doctype.name : null,
      compatMode: document.compatMode
    };
    document.getElementById("out").textContent =
      JSON.stringify(info, null, 2);
  </script>
</body>
</html>

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Document.doctype — vs compatMode (Try It)