第一种：定位
第一种思路：父元素relative，子元素absolute，left为50%，top为50%，再给子div设置距左和距上是自身的一半：transform:translate(-50%,-50%)。
代码实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.big-box {
width: 400px;
height: 400px;
background-color: crimson;
position: relative;
}
.small-box {
width: 150px;
height: 150px;
background-color: aqua;
position: absolute;
top: 50%;
left: 50%;
transform: translate(-50%, -50%);
}
</style>
</head>
<body>
<div class="big-box">
<div class="small-box"></div>
</div>
</body>
</html>第二种思路：父元素relative，子元素absolute，left为50%，top为50%，再给子div设置距左和距上是负自身的一半即：margin-left:负自身宽度/2，margin-top:负自身高度/2。
代码实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.big-box {
width: 400px;
height: 400px;
background-color: crimson;
position: relative;
}
.small-box {
width: 150px;
height: 150px;
background-color: aqua;
position: absolute;
top: 50%;
left: 50%;
margin-left: -75px;
margin-top: -75px;
}
</style>
</head>
<body>
<div class="big-box">
<div class="small-box"></div>
</div>
</body>
</html>第三种思路：父元素relative，子元素absolute，并且left，right，top，bottom设置为0，margin:auto 即可以实现水平垂直居中。
代码实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.big-box {
width: 400px;
height: 400px;
background-color: crimson;
position: relative;
}
.small-box {
width: 150px;
height: 150px;
background-color: aqua;
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
margin: auto;
}
</style>
</head>
<body>
<div class="big-box">
<div class="small-box"></div>
</div>
</body>
</html>第四种：flex布局
思路：给父元素设置display:flex，并且再给父元素设置水平居中：justify-content:center，垂直居中：align-items:center。这样，子元素就垂直水平居中啦！
代码实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.big-box {
width: 400px;
height: 400px;
background-color: crimson;
display: flex;
align-items: center;
justify-content: center;
}
.small-box {
width: 150px;
height: 150px;
background-color: aqua;
}
</style>
</head>
<body>
<div class="big-box">
<div class="small-box"></div>
</div>
</body>
</html>第五种：网格布局（grid）
思路：给父元素设置display:grid，并且再给父元素设置水平居中：justify-content:center，垂直居中：align-items:center。这样，子元素就垂直水平居中啦！(与flex相似)
代码实现：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.big-box {
width: 400px;
height: 400px;
background-color: crimson;
display: grid;
align-items: center;
justify-content: center;
}
.small-box {
width: 150px;
height: 150px;
background-color: aqua;
}
</style>
</head>
<body>
<div class="big-box">
<div class="small-box"></div>
</div>
</body>
</html>统一效果：
