<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
</head>
<body>

  <iframe id="iframe"></iframe>

  <script type="text/javascript">
    "use strict";
    addEventListener("load", () => {
      let iframe = document.getElementById("iframe");
      let doc = iframe.contentDocument;
      doc.open("text/html");
      doc.write("Hello.");
      doc.close();
    }, {once: true});
  </script>
</body>
</html>
