That is not valid xhtml so yes, Preview will tell you that.
Try this instead:
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops">
<head>
<title></title>
</head>
<body>
<pre>
0: (SWANK::DEBUG-IN-EMACS # < UNDEFINED-FUNCTION @ #x716b082a>)</pre>
</body>
</html>
Notice the "<" followed by an ">" both of which which is not allowed in xhtml. They must be xml encoded to "<" and ">" respectively.
Preview is showing you where the Chrome based browser is saying the xhtml is invalid.