The default template was not valid HTML 4.01 Transitional.
-default_main_header = """
-<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
+default_main_header = """\
+<?xml version="1.0" encoding="utf-8"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
+ "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
<head>
<link rel="alternate" title="%(title)s" href="%(fullurl)s/atom"
type="application/atom+xml" />
<head>
<link rel="alternate" title="%(title)s" href="%(fullurl)s/atom"
type="application/atom+xml" />
-<link href="%(css_url)s" rel="stylesheet"
- type="text/css" />
+<link href="%(css_url)s" rel="stylesheet" type="text/css" />
<title>%(title)s</title>
</head>
<title>%(title)s</title>
</head>