Here is a template I always use for cover.html and it validates!
PHP Code:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Cover</title>
<style type="text/css">
@page {
padding: 0;
margin: 0;
}
body {
text-align: center;
padding: 0;
margin: 0;
}
</style>
</head>
<body>
<div>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" height="100%" preserveAspectRatio="none" version="1.1" viewBox="0 0 563 751" width="100%">
<image height="751" width="563" xlink:href="../Images/cover.jpg"/>
</svg>
</div>
</body>
</html>