html----实现元素上下左右居中

<body>
  <div id="box">
    <div class="con"></div>
  </div>
</body>

方法1：

<style>
    #box {
       300px;
      height: 200px;
      background: lightpink;
      position: relative;
    }

    .con {
       100px;
      height: 100px;
      background: blue;
      position: absolute;
      left: 0;
      right: 0;
      top: 0;
      bottom: 0;
      margin: auto;
    }
  </style>

方法2：

<style>
    #box {
       300px;
      height: 200px;
      background: lightpink;
      position: relative;
    }

    .con {
       100px;
      height: 100px;
      background: blue;
      position: absolute;
      top: 50%;
      left: 50%;
      transform: translate(-50%, -50%);
    }
  </style>

方法3：

<style>
    #box {
       300px;
      height: 200px;
      background: lightpink;
      display: flex;
      justify-content: center;
      align-items: center;
    }

    .con {
       100px;
      height: 100px;
      background: blue;

    }
  </style>