加载loading效果，设置时间

<html>
<head>
<title>
asdf
</title>
<script>
i=0;
function a(){
i=i+1;
if(i<=1){
  document.getElementById("span1").innerHTML="<img src=http://www.siyuroom.com/images/loading.gif>";
  window.setTimeout("a()",3000);
}else{
  document.getElementById("span1").innerHTML="";
}
}
</script>
<body onload=a()>
<span id="span1">asfd</span>
</body>
</html>