var clip_t,clip_b,clip_r,clip_l,layer_t,layer_top,clip_top,clip_bottom,clip_left,clip_right;
function rollArea(top,right,bottom,left,string){
    var HTMLStr;
    layer_t=layer_top=bottom;
    clip_t=clip_top=top-bottom;
    clip_b=clip_bottom=0;
    clip_l=clip_left=0;
    clip_r=clip_right=right-left;
    HTMLStr="<div style=position:absolute;background-color:blue;filter:alpha(opacity=20)"+
        ";top:"+top+"px;left:"+left+
        "px;width:"+(right-left)+"px;height:"+(bottom-top)+" ></div>";
    HTMLStr+="<div id=layer style=visibility:hidden;position:absolute";
    HTMLStr+=";top:"+bottom+"px;left:"+left+"px;width:"+(right-left)+
        "px;rect("+clip_t+"px "+clip_r+"px "+clip_b+"px "+clip_l+"px) >";
    HTMLStr+="</div>";
    document.write(HTMLStr);
    //layer.innerText=HTMLStr;
    layer.innerHTML=string;
    layer.style.visibility="visible";
    //;zIndex:-1
    //layer.style.zIndex=1;
}
function rollDida(){
    clip_t++;
    clip_b++;
    layer_t--;
    if(clip_t>parseInt(layer.clientHeight)){
        clip_t=clip_top;
        clip_b=clip_bottom;
        layer_t=layer_top;
    }
    layer.style.clip="rect("+clip_t+"px "+clip_r+"px "+clip_b+"px "+clip_l+"px)";
    layer.style.top=layer_t+"px";
}

用法：
<script language="javascript" src=../../"/hangzhou/include/rollarea.js"></script>
<script language="javascript">
<!--
    rollArea(0,570,40,220,"你的文本");
    window.setInterval("rollDida()",100);
//-->
</script>