(JS) Browser Detect (Netscape - Mozilla)

[MasterT]

Hey im working on a browser detect script for my .css imports here is the script

<script type="text/javascript" language="javascript"> 
BrowserDescription = function () { 
    this.appN = navigator.appName.toLowerCase(); 
    this.appV = parseInt(navigator.appVersion); 
    this.ua = navigator.userAgent.toLowerCase(); 
    this.plt = navigator.platform.toLowerCase(); 
    if (this.ua.indexOf('opera/7') != -1 || this.ua.indexOf('opera 7') != -1) { 
        this.appV = 7; 
    } 
    if (this.appN.indexOf('netscape') != -1) { 
        document.write("<link href=SignsNS.css rel=stylesheet type=text/css media=screen>"); 
    } 
    if (this.appN.indexOf('microsoft internet explorer') != -1) { 
        document.write("<link href=Signs.css rel=stylesheet type=text/css media=screen>"); 
    }     

} 
var browser = new BrowserDescription(); 
</script>

here is the results for Mozilla
this.appN(Name) = netscape
this.appV(Version) = 5
this.ua(userAgent) = mozilla/5.0 (windows; u; winnt4.0; en-us; rv:1.6) gecko/20040113
this.plt(Platform) = win32

and Netscape
this.appN(Name) = netscape
this.appV(Version) = 5
this.ua(userAgent) = mozilla/5.0 (windows; u; winnt4.0; en-us; rv:1.4) gecko/20030624 netscape/7.1 (ax)
this.plt(Platform) = win32

so im guessing that i'd have to use this.ua to tell the difference between the both but then there is a problem with the versions cause i could put
if {this.ua = mozilla/5.0 (windows; u; winnt4.0; en-us; rv:1.6) gecko/20040113
then use mozilla css
if {this.ua = mozilla/5.0 (windows; u; winnt4.0; en-us; rv:1.4) gecko/20030624 netscape/7.1 (ax)}
then use netscape css

is there a better way?

[carbonize]

Surely if they use the Gecko engine then they will render pages the same and so you can use the same CSS for both?

If ua contains Opera it's Opera, If it contains Gecko it uses Gecko else it uses the IE engine (mosaic?). Oh and of course there's the Safari rendering engine.