1、实现原理
* 实际上弹出层、遮罩层和原页面显示分别为三个不同的div
* 弹出层的层级在遮罩层之上，遮罩层的层级在原页面显示之上；
* 遮罩层有通明效果
* 遮罩层没有实际意义，则无需在html部分就写上，当然写上同样可以实现
2、代码实现
html语言如下：
<body>
<center>
<div ><input type="button" value="go" onclick="show()"></div>
<div id="alert" style="display:none;">
<form>
登录
<input type="text"><input type="password"><input type="submit" value="login">
</form>
</div>
</center>
</body>
javascript实现弹出层和遮罩层：
<span style="font-size:12px;"><script>function show(){
var alertPart=document.getElementById("alert");
alertPart.style.display="block";
alertPart.style.position = "absolute";
alertPart.style.top = "50%";
alertPart.style.left = "50%";
alertPart.style.marginTop = "-75px";
alertPart.style.marginLeft = "-150px";
alertPart.style.background="cyan";
alertPart.style.width="300px";
alertPart.style.height="200px";
alertPart.style.zIndex = "501";
var mybg = document.createElement("div");
mybg.setAttribute("id","mybg");
mybg.style.background = "#000";
mybg.style.width = "100%";
mybg.style.height = "100%";
mybg.style.position = "absolute";
mybg.style.top = "0";
mybg.style.left = "0";
mybg.style.zIndex = "500";
mybg.style.opacity = "0.3";
mybg.style.filter = "Alpha(opacity=30)";
document.body.appendChild(mybg);
document.body.style.overflow = "hidden";
}
</script></span>