高分求救：这个效果如何实现？

<BODY>
<span id="hwtext" style="FILTER: alpha(opacity=100);height:20px;">222</span>
<SCRIPT LANGUAGE="JavaScript">
<!--
var highlighting;
function lownow(which2)
{
theobject=which2
highlighting=setInterval("low(theobject)",100)
}
function low(cur2)
{
if(cur2.filters.alpha.opacity>0)
{
cur2.filters.alpha.opacity-=10
}
else
{
clearInterval(highlighting)
}
}//-->
</SCRIPT>
<button onclick='lownow(hwtext)'>确定</button>
</BODY>