设置div中的div居中显示

设置div中的div居中显示

方法一、

<div class='big'>

    <div class='small'>box1</div>

 </div>

style样式：
.big{
  height:200px;
  width:200px;
  border:black solid 1px;
  position:absolute;
  left:150px;

}
.small{
  height:100px;
  width:100px;
  background-color:green;
  position:relative;
  left:100px;
  top:100px;
  margin-top:-50px;
  margin-left:-50px;
  border:black solid 1px;
}

方法二、

div class='big2'>

    <div class='small2'>box3</div>

 </div>

.big2{
  height:200px;
  width:200px;
  border:black solid 1px;
  text-align:center;

}
.small2{
  height:100px;
  width:100px;
  background-color:green;
  margin:50px auto; //外面的div高度的一半
  border:black solid 1px;
}