HTML弹出层，始终显示在屏幕正中间

1.让层始终显示在屏幕正中间： 
CSS代码： 
```css
<style type="text/css">  
    .mask {    
            position: absolute; top: 0px; filter: alpha(opacity=60); background-color: #777;  
            z-index: 1002; left: 0px;  
            opacity:0.5; -moz-opacity:0.5;  
        }  
    .model{  
            position: absolute; z-index: 1003;   
            width:320px; height:320px; text-align:center;  
            background-color:#0066FF; display: none;  
        } 
</style>
```
jquery代码： 
```javascript
<script type="text/javascript">  
    //兼容火狐、IE8  
    function showMask(){  
        $("#mask").css("height",$(document).height());  
        $("#mask").css("width",$(document).width());  
        $("#mask").show();  
    }  
    //让指定的DIV始终显示在屏幕正中间  
    function letDivCenter(divName){   
        var top = ($(window).height() - $(divName).height())/2;   
        var left = ($(window).width() - $(divName).width())/2;   
        var scrollTop = $(document).scrollTop();   
        var scrollLeft = $(document).scrollLeft();   
        $(divName).css( { position : 'absolute', 'top' : top + scrollTop, left : left + scrollLeft } ).show();  
    }  
    function showAll(divName){  
        showMask();  
        letDivCenter(divName);  
    }  
</script>
```
HTML代码： 
```html
<div id="mask" class="mask"></div>  
<a href="javascript:;" onclick="showMask()" >点我显示遮罩层</a><br />  
<a href="javascript:;" onclick="letDivCenter('#model')">点我让DIV始终显示在屏幕中间</a><br />  
<a href="javascript:;" onclick="showAll('#model')">点我显示所有</a><br />  
<div>  
    <div id="model" class="model">  
        这是内容 
    </div>  
</div>
```

Search result.

html固定在屏幕下方不滚动的div

CSS 的优先级机制