实现垂直居中的七种方式1. table 自带功能 2. 将 div 改造成 table 3. 100% 高度的 afte

1. table 自带功能

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>table 自带功能</title>
  <style>
    .parent {
      border: 1px solid red;
      height: 600px;
     }

     .child {
       border: 1px solid green;
     }
  </style>
</head>
<body>
  <table class="parent">
    <tr>
      <td class="child">
      一段文字
      </td>
    </tr>
  </table>
</body>
</html>

2. 将 div 改造成 table

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>将 div 改造成 table</title>
  <style>
    div.table {
      display: table;
      border: 1px solid red;
      height: 600px;
    }

    div.tr {
      display: table-row;
      border: 1px solid green;
    }

    div.td {
      display: table-cell;
      border: 1px solid blue;
      vertical-align: middle;
    }

    .child {
      border: 10px solid black;
    }
  </style>
</head>
<body>
  <div class="table">
      <div class="td">
        <div class="child">
      一段文字
     </div>
    </div>
  </div>
</body>
</html>

3. 100% 高度的 after before 加上 inline block

.parent{
  border: 3px solid red;
  height: 600px;
  text-align: center;
}

.child{
  border: 3px solid black;
  display: inline-block;
  width: 300px;
  vertical-align: middle;
}

.parent:before{
  content:'';
  outline: 3px solid red;
  display: inline-block;
  height: 100%;
  vertical-align: middle;
}
.parent:after{
  content:'';
  outline: 3px solid red;
  display: inline-block;
  height: 100%;
  vertical-align: middle;
}

4. margin-top: -50%

.parent{
  height: 600px;
  border: 1px solid red;
  position: relative;
}
.child{
  border: 1px solid green;
  width: 300px;
  position: absolute;
  top: 50%;
  left: 50%;
  margin-left: -150px;
  height: 100px;
  margin-top: -50px;
}

5. translate: -50%

.parent{
  height: 600px;
  border: 1px solid red;
  position: relative;
}
.child{
  border: 1px solid green;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%,-50%);
}

6. absolute margin auto

.parent{
  height: 600px;
  border: 1px solid red;
  position: relative;
}
.child{
  border: 1px solid green;
  position: absolute;
  width: 300px;
  height: 200px;
  margin: auto;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

7. flex

.parent{
  height: 600px;
  border: 3px solid red;
  
  display: flex;
  justify-content: center;
  align-items: center;
}
.child{
  border: 3px solid green;
  width: 300px;
}

总结：

如果 .parent 的 height 不写，则只需要 padding: 10px 0; 就能将 .child 垂直居中
如果 .parent 的 height 写死了，就需要用到上述几种方法把 .child 居中
建议：能不写 height 就千万别写 height