<!DOCTYPE html>
<html>
<head>
  <meta charset=utf-8>
  <title>&lt;sub&gt;</title>
</head>
<body>
  <!-- is structured inline -->
  <p><sub class="class" lang="en">text</sub></p>

  <!-- is strictly inline -->
  <p><dfn><sub class="class" lang="en">text</sub></dfn></p>

  <!-- can be empty -->
  <p>text <sub></sub></p>
  <p>text <dfn><sub></sub></dfn></p>

  <!-- can contain interactive -->
  <p><sub><a>text</a></sub></p>
  <p><dfn><sub><a>text</a></sub></dfn></p>

  <!-- cannot contain structured inline -->
</body>
</html>
