JavaScript实现多层级Div显示与隐藏控制

<html>
<head>
<title>中国风</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head>
<script language="JavaScript">
function showLay1(){
  lay1.style.visibility="visible";
  lay2.style.visibility="hidden";
  lay3.style.visibility="hidden";
}
function showLay2(){
  lay1.style.visibility="visible";
  lay2.style.visibility="visible";
  lay3.style.visibility="hidden";
}
function showLay3(){
  lay1.style.visibility="visible";
  lay2.style.visibility="visible";
  lay3.style.visibility="visible";
}
</script>
<body>
<div id="lay1" style="position:absolute; left:60px; top:30px; width:250px; height:200px; index:1; visibility:visible">这是第一层内容,它的背景是透明的</div>
<div id="lay2" style="position:absolute; left:60px; top:30px; width:250px; height:200px; index:2; visibility:hidden; background-color:silver">这是第二层内容,它的背景是灰色的</div>
<div id="lay3" style="position:absolute; left:60px; top:30px; width:250px; height:200px; index:3; visibility:hidden"><br><br><br>这是第三层内容,它的背景是透明的</div>
<form id="form1" style="position:absolute; left:70px; top:250px;">
<input type="button" value="第一层" onclick="showLay1()">
<input type="button" value="第二层" onclick="showLay2()">
<input type="button" value="第三层" onclick="showLay3()">
</form>
</body>
</html>