if (screen.width <= 719) { 
document.write('<link rel="stylesheet" href="css/milo800.css">'); 
} 

if ((screen.width >= 720) && (screen.width <= 920)) { 
document.write('<link rel="stylesheet" href="css/milo800.css">'); 
} 
if (screen.width >= 921) { 
document.write('<link rel="stylesheet" href="css/milo.css">'); 
}