How does JS determine the mobile access device and parse the corresponding CSS

 
//Determine whether it is a mobile operating environment
if(/AppleWebKit.*Mobile/i.test(navigator.userAgent) || (/MIDP|SymbianOS|NOKIA|SAMSUNG|LG|NEC|TCL|Alcatel|BIRD|DBTEL|Dopod|PHILIPS|HAIER|LENOVO|MOT-|Nokia|SonyEricsson|SIE-|Amoi|ZTE/.test(navigator.userAgent))){ 
if(window.location.href.indexOf("?mobile")<0){ 
try{ 
if(/Android|webOS|iPhone|iPod|BlackBerry/i.test(navigator.userAgent)){ 
//If the access environment is Android|, webOS, |, iPhone, |, iPod, |, BlackBerry, then load the following styles
setActiveStyleSheet("style_mobile_a.css"); 
} 
else if(/iPad/i.test(navigator.userAgent)){ 
//If the access environment is iPad, load the following styles
setActiveStyleSheet("style_mobile_iPad.css"); 
} 
else{ 
//Determining that the access environment is another mobile device loads the following styles
setActiveStyleSheet("style_mobile_other.css"); 
} 
} 
catch(e){} 
} 
} 
else{ 
//If none of the above, load the following styles
setActiveStyleSheet("style_mobile_no.css"); 
} 
//After determining the loading style
function setActiveStyleSheet(filename){document.write("<link href="+filename+" rel=stylesheet>");}