Css should not relate to the main page, but rather should relate to the location of the page instance using the main page. In most cases, this will be the same thing, but I always try to use either the full path or the relative path of the site
Fully qualified way
<link href="http://some.site.com/mysite/styles/default.css" rel="stylesheet" type="text/css" />
or relative path (note that this may not work if you have a version that can host only one site, but many applications such as WinXP)
<link href="/default.css" rel="stylesheet" type="text/css" />
Win relative xp path
<link href="/path/to/application/default.css" rel="stylesheet" type="text/css" />
Mauro source share