<!DOCTYPE html>
<html>
<head>
  <meta charset=utf-8>
  <title>&lt;sub&gt;</title>
</head>
<body>
  <!-- not a block -->
  <sub>text</sub>

  <!-- cannot contain structured inline -->
  <p><sub><ul><li>text</li></ul></sub></p>

  <!-- cannot contain interactive if parent forbids interactive -->
  <p><a><sub><a>text</a></sub></a></p>
  <p><a><dfn><sub><a>text</a></sub></dfn></a></p>
</body>
</html>