Access: [6.2.2.1]: text equivalents require updating (applet)
  Priority 1

Cause:

Whenever dynamic content changes, text equivalents must also be updated. If an applet is used to generate dynamic content, provide a text equivalent between the opening and closing applet elements or via the "alt" attribute.

Testfile:


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
<html lang="en">
<head>
<title>aert1.0/6.2.2</title>
</head>
<body>
<applet code="applet.class"></applet>
</body>
</html>


References:

W3C WAI: http://www.w3.org/WAI//

HTML Techniques for WAI 1.0: http://www.w3.org/TR/WCAG10-HTML-TECHS/