div+css 怎么让一个小div在另一个大div里面 垂直居中

.parent {
          width:800px;
          height:500px;
          border:2px solid #000;
          position:relative;
}
 .child {
            width:200px;
            height:200px;
            margin: auto;  
            position: absolute;  
            top: 0; left: 0; bottom: 0; right: 0; 
            background-color: red;
}

.parent {
            width:800px;
            height:500px;
            border:2px solid #000;
            display:table-cell;
            vertical-align:middle;
            text-align: center;
}
.child {
            width:200px;
            height:200px;
            display:inline-block;
            background-color: red;
}

.parent {
            width:800px;
            height:500px;
            border:2px solid #000;
            position:relative;
}
.child {
            width:300px;
            height:200px;
            margin:auto;
            position:absolute;/*设定水平和垂直偏移父元素的50%，
再根据实际长度将子元素上左挪回一半大小*/
            left:50%;
            top:50%;
            margin-left: -150px;
            margin-top:-100px;
            background-color: red;
}