if(document.layers){ 
document.write("<link rel='stylesheet' href='http://www.html.it/stilens.css' type='text/css'>"); 
} 
if(document.all){ 
document.write("<link rel='stylesheet' href='http://www.html.it/stile.css' type='text/css'>"); 
}
if(document.getElementById){
document.write("<link rel='stylesheet' href='http://www.html.it/stilenn6.css' type='text/css'>");
}