方法1：.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;}方法2：.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;        }方法3：.parent {
               width:800px;            height:500px;            border:2px solid #000;            display:flex;            justify-content:center;            align-items:center;        }        .child {
               width:200px;            height:200px;            background-color: red;        }方法4：.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;        }