<html xmlns="http://www.w3.org/1999/xhtml">
<head>
  <title>&lt;iframe&gt;</title>
</head>
<body>
  <!-- is structured inline -->
  <p><iframe
      src="uri"
      class="class"
      xml:lang="en"></iframe></p>

  <!-- is strictly inline -->
  <p><dfn><iframe
      src="uri"
      class="class"
      xml:lang="en"></iframe></dfn></p>

</body>
</html>