Access: [1.1.8.1]: <frame> may require 'longdesc'
(Priority 1)

Cause:

If a frame cannot be adequately described in its title, include the "longdesc" attribute within the frame element to provide a link to additional information. The "longdesc" attribute can also be useful in helping to explain relationships between frames.

Testfile:

<html lang="en">
<head><title>Frame elements missing valid long descriptions</title></head>
<frameset rows="100,200">
 <frame src="TableData.htm">
 <frame src="TableLayout.htm" title="this frame title" longdesc="tl.htm">
 <frame src="TableLayout2.htm" title="this frame title" longdesc="tl2.htm">
</frameset>
</html>

References:

W3C WAI: http://www.w3.org/WAI//
HTML Techniques for WAI 1.0: http://www.w3.org/TR/WCAG10-HTML-TECHS/