弹出层透明背景加框的常用样式和结构

.alertMessageBg{
position:fixed;
_position:absolute;
width:100%;
height:100%;
left:0;
top:0;
background:#000;
opacity:0.5;
-moz-opacity:0.5;
filter:alpha(opacity=50);
z-index:97;
display:none;
}
.alertMessageDivBorder{
position:fixed;
_position:absolute;
width:750px;
height:370px;
left:50%;
top:50%;
margin:-185px 0 0 -375px;
background:#000;
filter:alpha(opacity=30);
-moz-opacity:0.3;
opacity:0.3;
z-index:98;
display:none;
}
.alertMessageDiv{
position:fixed;
_position:absolute;
width:730px;
height:350px;
left:50%;
top:50%;
margin:-175px 0 0 -365px;
background:#fff;
z-index:99;
display:none;
font-size:14px;
}

<div class="alertMessageBg"></div>
<div class="alertMessageDivBorder"></div>
<div class="alertMessageDiv"></div>

弹出层背景覆盖整个网页的方法

1.最近突然想起用的css方法

.alertMessageBg{
position:fixed;
_position:absolute;
width:100%;
height:100%;
left:0;
top:0;
background:#000;
opacity:0.5;
-moz-opacity:0.5;
filter:alpha(opacity=50);
z-index:97;
display:none;
}

2.js方法

.alertMessageBg{
position:absolute;
width:100%;
height:100%;
left:0;
top:0;
background:#000;
opacity:0.5;
-moz-opacity:0.5;
filter:alpha(opacity=50);
z-index:97;
display:none;
}

var bgWidth = document.body.clientWidth + ‘px‘,
　　bgHeight = document.body.clientHeight + ‘px‘,
　　alertBgNode = $(‘.alertMessageBg‘);
alertBgNode.css({‘width‘:bgWidth,‘height‘:bgHeight});

对比以上两种方法，显然css方法更省事，尤其在不用兼容ie6的现在。哈哈