实现方法有三种
方法一 利用 display:table-cell 设置单元格的属性
思路:
先给父盒子设置table-cell 设置摆放位置
再设置 vertical-align: middle 摆放在中间
然后给父盒子设置内联块属性display: inline-block
最后父元素就可以用文字居中的属性来实现子盒子的水平居中
就达成了盒子水平垂直居中了。
代码如下:
<!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>Document</title>
<style>
.parent {
width: 500px;
height: 500px;
border: 1px solid red;
display: table-cell;
vertical-align: middle;
text-align: center;
}
.child {
width: 100px;
height: 100px;
border: 1px solid #000;
display: inline-block;
}
</style>
</head>
<body>
<div class="parent">
<div class="child">子元素</div>
</div>
</body>
</html>
如图所示:

方法二:利用 display:flex;flex布局 设置垂直水平居中
flex布局 给弹性盒子设置 display: flex;
设置子元素的主轴 justify-content: center; 水平方向居中
设置子元素的侧轴 align-items: center; 垂直方向居中
代码如下
<!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>Document</title>
<style>
.parent {
margin: 100px auto;
width: 500px;
height: 500px;
border: 5px solid pink;
display: flex;
justify-content: center;
align-items: center;
}
.child {
width: 100px;
height: 100px;
border: 5px solid skyblue;
text-align: center;
}
</style>
</head>
<body>
<div class="parent">
<div class="child">子元素</div>
</div>
</body>
</html>
如图所示:

方法三:计算父盒子与子盒子的空间距离(这跟方法一是一个道理)
<!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>Document</title>
<style>
.parent {
margin: 100px auto;
width: 500px;
height: 500px;
border: 5px solid yellow;
}
.child {
width: 100px;
height: 100px;
border: 5px solid skyblue;
text-align: center;
margin-top: 200px;
margin-left: 200px;
}
</style>
</head>
<body>
<div class="parent">
<div class="child">子元素</div>
</div>
</body>
</html>
如图所示:
