<!DOCTYPE html>
<html>
<head>
  <meta charset=utf-8>
  <title>&lt;h1&gt;</title>
</head>
<body>
  <!-- is block -->
  <h1 class="class" id="id" lang="en">text</h1>

  <!-- can contain strictly inline -->
  <h1>text <em>elem</em></h1>

  <!-- can contain interactive -->
  <h1><a>text</a></h1>
</body>
</html>