简洁倒计时脚本程序

<script>
i=100
function aa()
{if(i>0)
i--
a.innerText=i}
</script>
<button onclick=setInterval("aa()",100) style="border:0px; background:#FF9900; height:25px; font-weight:bold; color:#c00; float:left; cursor:pointer;">开始倒计时</button>
<div id="a" style="color:#FF0000; font-weight:bold; float:left; height:30px;">100</div>