传统定位和flex定位 -

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>元素居中的两种方法</title>
    <style>
      .box {
        width: 15em;
        height: 15em;
        background-color: crimson;
        position: relative;
      }
      .box .sun {
        width: 5em;
        height: 5em;
        background-color: darkblue;
        position: absolute; /**绝对定位*/
        top: 0;
        left: 0;
        right: 0;
        bottom: 0; /**top left right bottom  等于0让盒子居中*/
        margin: auto; /*这一行才做了sun 元素再box中的居中效果，没有的话无效果*/
      }
      .box1 {
        width: 15em;
        height: 15em;
        background-color: darkorange;
        display: flex;
        justify-content: center;
        align-items: center;
        /** display: flex;justify-content: center; align-items: center; 完成了利用flex居中效果*/
      }
      .box1 .sun1 {
        width: 5em;
        height: 5em;
        background-color: darkslateblue;
      }
    </style>
  </head>
  <body>
    <h2>传统居中</h2>
    <div class="box">
      <div class="sun"></div>
    </div>
    <h2>flex居中</h2>
    <div class="box1">
      <div class="sun1"></div>
    </div>
  </body>
</html>