基于JS实现弹出一个隐藏的div窗口body页面变成灰

/ 定义一个div用于覆盖整个页面，这个div的z-index大于body，小于服务评分div /
#temp{
  background-color: #000;
  opacity: 0.3;
  width: 100%;
  height: 100%;
  z-index: 2;
  visibility: hidden;
}
/ 评分div，其z-index最大(当显示时) /
#mark{
 background-color: rgb(255,255,255);
 width: 400px;
 height: 250px;
 z-index: 3;
 visibility: hidden;
}
/ 包含了服务评分、购物车等的div /
#main{
 background-color: rgb(255,255,255);
 width: 100%;
 height: 100%;
 z-index: 1;
}
<div id="temp"></div>
<div id="mark"></div>
<div id="main"></div>
<a id="link"><i>点击我进行层叠</a>
<script>
 window.onload = function() {
  document.getElementById("link").onclick = function(){
   document.getElementById("temp").style.visibility = "visible";
   document.getElementById("mark").style.visibility = "visible";
  }
 }
</script>