请问如何在浏览器中中判断用户按了“STOP”按钮？

停止有document.onstop，看微软的例子<body scroll=no>
<script>
document.onstop=fnTrapStop;
var oInterval;
window.onload=fnInit;
function fnInit(){
   oInterval=window.setInterval("fnCycle()",1);
}
function fnCycle(){
   // Do something
}
function fnTrapStop(){
   window.clearInterval(oInterval);
   alert();
}
</script>
</body>