HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>HTML contentType check</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var isHtml = document.contentType === "text/html";
    document.getElementById("out").textContent = [
      "Is HTML document: " + isHtml,
      "contentType: " + document.contentType
    ].join("\n");
  </script>
</body>
</html>

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Document.contentType — HTML check (Try It)