if (navigator.appName.indexOf('Microsoft') == -1)
{
    if (navigator.appVersion.substr(0, 1) < 5)
    {
        document.write('<link rel="stylesheet" type="text/css" href="styles/ns4_styles.css">');
    }
    else
    {
        document.write('<link rel="stylesheet" type="text/css" href="styles/ns6_styles.css">');
    }
}
else
{
    document.write('<link rel="stylesheet" type="text/css" href="styles/ie_styles.css">');
}