var style = document.createElement("STYLE");
 document.body.appendChild(style);
 style.onerror = function(){
  alert("here1")
 }
 style.onreadystatechange = function(){
  alert("here2");
 }
var sheet = style.styleSheet || style.sheet;

if(sheet.addImport)
 sheet.addImport("aa.css"); 
else 
 sheet.insertRule("@import url('aa.css');",sheet.cssRules.length);