programmatic-focus-iframe.html

<!DOCTYPE html>
<html>
<body>
  <h1>Iframed Page</h1>
  <p>This page will steal focus after the countdown</p>
  <p id="countdown">3</p>
  <input id="input" type="text">
  <script>
    window.onfocus = () => {
      document.getElementById("input").focus();
    };

    var countdown = 3;
    var interval = setInterval(function () {
      countdown--;
      document.getElementById("countdown").textContent = countdown;
      if (countdown === 0) {
        clearInterval(interval);
        document.getElementById("input").focus();
      }
    }, 1000);
  </script>
  <style>
    body {
      font-family: Arial, sans-serif;
    }
    input {
      font-size: 1.5em;
      padding: 0.5em;
      width: 50%;
    }
    input:focus {
      outline: 2px solid blue;
    }
  </style>
</body>
</html>