一个div相对于外层的div水平和垂直居中

我自己感觉，第四种比较常用

<title>无标题文档</title>
<style>
        .parent {
          800px;
          height:500px;
          border:2px solid #000;
          position:relative;
          }
         .child {
            200px;
            height:200px;
            margin: auto;
            position: absolute;
            top: 0; left: 0; bottom: 0; right: 0;
            background-color: red;
         }


        .parent1 {
            800px;
            height:500px;
            border:2px solid #000;
            display:table-cell;
            vertical-align:middle;
            text-align: center;
        }
        .child1{
            200px;
            height:200px;
            display:inline-block;
            background-color: red;
        }


  .parent2{
            800px;
            height:500px;
            border:2px solid #000;
            display:flex;
            justify-content:center;
            align-items:center;
        }
        .child2{
            200px;
            height:200px;
            background-color: red;
        }



   .parent3 {
            800px;
            height:500px;
            border:2px solid #000;
            position:relative;
        }
        .child3 {
            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;
        }

</style>

</head>

<body>
<div class="parent">
<div class="child">第一种方法</div>
</div>

<div class="parent1">
<div class="child1">第二种方法</div>
</div>

<div class="parent2">
<div class="child2">第三种方法</div>
</div>

<div class="parent3">
<div class="child3">第四种方法</div>
</div>

</body>
</html>