You probably just need a valid URL DOCTYPE, add the highlighted as the very first thing in your source code as shown:
Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<link rel="stylesheet" href="lightbox.css" type="text/css" media="screen">
<script type="text/javascript" src="prototype.js"></script>
<script type="text/javascript" src="scriptaculous.js?load=effects,builder"></script>
<script type="text/javascript" src="lightbox.js"></script>
</head>
. . .
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