Access: [6.2.1.1]: <frame> source invalid
(Priority 1)

Cause:

Make the source of each frame a valid html document. If an image file is used as the source of a frame, you will not be able to include "alt" text with the graphic. Also, do not use the "title" attribute to describe the contents of a frame. If the content changes, the "title" will no longer be applicable.

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>aert1.0/6.2.1</title>
</head>
<body>
<frame src="rex.jpg">
</body>
</html>

References:

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