HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>loading timeline</title>
</head>
<body>
  <pre id="out">Listening...</pre>
  <script>
    var out = document.getElementById("out");
    var lines = [];
    function log(msg) {
      lines.push(msg);
      out.textContent = lines.join("\n");
    }

    document.addEventListener("readystatechange", function () {
      log("readystate: " + document.readyState);
    });

    document.addEventListener("DOMContentLoaded", function () {
      log("DOMContentLoaded");
    });

    window.addEventListener("load", function () {
      log("load");
    });
  </script>
</body>
</html>

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Document DOMContentLoaded — loading timeline (Try It)