如果 .parent 的 height 不写，你只需要 padding: 10px 0; 就能将 .child 垂直居中；
如果 .parent 的 height 写死了，就很难把 .child 居中，以下是垂直居中的方法。
忠告：能不写 height 就千万别写 height。

1、table自带功能

代码

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <table class="parent"> 
    <tr>
      <td class="child">
      【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
      </td>
    </tr>
  </table>
</body>
</html>

.parent{
  border: 1px solid red;
  height: 600px;
}

.child{
  border: 1px solid green;
}

效果图

2、100% 高度的 afrer before 加上 inline block

代码

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div class="parent">
    <span class=before></span><div class="child">
      【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
    </div><span class=after></span>
  </div>
</body>
</html>

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

效果图
在这里插入图片描述

优化版

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div class="parent">
    <div class="child">
      【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
    </div>
  </div>
</body>
</html>

.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;
}

3、div 装成 table

代码

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div class="table">
      <div class="td">
        <div class="child">
           【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
        </div>
    </div>
  </div>
</body>
</html>

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;
}

效果图
在这里插入图片描述

4、margin-top -50%

代码

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div class="parent">
    <div class="child">
     【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
    </div>
  </div>
</body>
</html>

.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;
}

效果图
在这里插入图片描述

5、translate -50%

代码

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div class="parent">
    <div class="child">
      【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
    </div>
  </div>
</body>
</html>

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

效果图

6、absolute margin auto

代码

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div class="parent">
    <div class="child">
        【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
    </div>
  </div>
</body>
</html>

.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;
}

效果图
在这里插入图片描述

7、flex

代码

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JS Bin</title>
</head>
<body>
  <div class="parent">
    <div class="child">
       【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】【垂直居中】
    </div>
    
  </div>
</body>
</html>

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