HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>featurePolicy lists</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var out = document.getElementById("out");
    if (!("featurePolicy" in document)) {
      out.textContent = "document.featurePolicy not available";
    } else {
      var fp = document.featurePolicy;
      var supported = fp.features();
      var allowed = fp.allowedFeatures();
      out.textContent =
        "supported count: " + supported.length + "\n" +
        "allowed count: " + allowed.length + "\n" +
        "sample allowed: " + allowed.slice(0, 5).join(", ");
    }
  </script>
</body>
</html>

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features() and allowedFeatures() (Try It)