Access: [1.1.1.10]: suspicious 'alt' text (too long)
  Priority 1

Cause:

All images require text equivalents but "alt" text must also meet certain criteria otherwise it may be considered suspicious. If "alt" text exceeds 150 characters, use the "longdesc" attribute along with a descriptive link (d-link) to describe the image.

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>aert1.0/1.1.1</title>
</head>
<body>
<img src="hasAlt.gif" alt="0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789 0123456789"/>
</body>
</html>


References:

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

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