css中怎么设置居中 css怎么设置位置居中

<div class="img_wrap">
 　　<img src="wgs.jpg">
</div>

.img_wrap{
    width: 400px;
    height: 300px;
    border: 1px dashed #ccc;
    display: table-cell; //主要是这个属性
    vertical-align: middle;
    text-align: center;
}

<div class="img_wrap"></div>

.img_wrap{
    width: 400px;
    height: 300px;
    border: 1px dashed #ccc;
    background: url(wgs.jpg) no-repeat center center;
}

 <div class="img_wrap">
     <p><img src="wgs.jpg"></p>
 </div>

*{margin: 0px;padding: 0px}
.img_wrap{
    width: 400px;
    height: 300px;
    border: 1px dashed #ccc;
    text-align: center;}
.img_wrap p{
    width:400px;
    height:300px;
    line-height:300px;  
}
.img_wrap p img{
    *margin-top:expression((400 - this.height )/2);  
    vertical-align:middle;
    border:1px solid #ccc;
}