Html遮罩层的显示(主要在于样式设置)

<html>
<head>
<title>aaa</title>
<script type="text/javascript">
    window.onload = function () {
        document.getElementById("divshow").style.display = "none";
    }
    function btnclick() {
        document.getElementById("divshow").style.display = "block";
    }
</script>
<style type="text/css">
body
{
  margin:0px;
  padding:0px; 
}
#divshow
{
    position: fixed;  /*相对于浏览器窗口进行定位*/
    *position:absolute;
    left: 0px;
    top: 0px;
    width: 100%;
    height: 100%;     
     /* 透明度*/
    display:none;
    z-index:999; 
}
#divGary
{
    width:100%;height:100%;background:#000;opacity:0.5;filter:alpha(opacity=50);position:absolute;left:0px;top:0px; z-index:1;  
}
</style>
</head>
<body>
     <input type="button" id="btnShow" onclick="btnclick()" value="显示遮罩" />
  <div id="divshow">
            <div id="divGary" ></div>    <!--后面的遮罩-->
           <div style=" width:500px; height:312px; left:50%; top:50%; position:relative;z-index:2; background-color:#fff; margin-left:-150px; margin-top:-100px;border-radius:5px;" >  <!--显示层-->
              <div >
                    <div style=" text-align: center;color: red;font-weight: bold;font-size: 21px;"><p style="padding-top: 30px;">你妹</p></div>  
                    <div style="text-indent: 2em;width: 400px;margin: 16px auto;line-height: 2;font-size: 18px;font-weight: bold;">你妹的具体内容</div>
              </div>
              <div style="width: 228px;margin: -14px auto; margin-left:117px;">
                  <input id="btngobuy" class="input_but" type="button" value="找你妹" style=" display:none;" />
                  <input id="btnnext" class="input_but" style="margin-left: 71px;"  type="button" value="不找你妹了" />
              </div>
           </div>
        </div>
</body>
</html>