概述
水平垂直居中对齐
方案
水平居中：
inline 元素： text-align: center
block 元素： margin：auto
absolute元素：left: 50% + margin-left 负值
垂直居中：
inline 元素：line-height 的值等于height值
absolute 元素：top: 50% + margin-top 负值(需要知道子元素的宽高)
absolute 元素：transform(-50%,-50%)
absolute 元素：top left bottom right = 0 + margin: auto
水平居中：
inline 元素： text-align: center
block 元素： margin：auto
absolute元素：left: 50% + margin-left 负值
absolute 元素：left: 50% + transform:translate(-50%, -50%)
absolute 元素：top left bottom right = 0 + margin: auto
display: flex, jusitity-content: center
垂直居中：
inline 元素：line-height 的值等于height值
absolute 元素：top: 50% + margin-top 负值(需要知道子元素的宽高)
absolute 元素：top: 50% + transform:translate(-50%, -50%)
absolute 元素：top left bottom right = 0 + margin: auto
display: flex; align-items: center;
absolute 元素：top left bottom right = 0 + margin: auto 居中原理
关键在于 margin: auto ，它能自动填充外边距。
就跟设置了 width 然后设置 margin: 0 auto; 能水平居中，一样
示例
水平居中
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>水平对齐</title>
<style type="text/css">
.container {
border: 1px solid #ccc;
margin: 10px;
padding: 10px;
}
.item {
background-color: #ccc;
}
.container-1 {
text-align: center;
}
.container-2 .item {
width: 500px;
margin: auto;
}
.container-3 {
position: relative;
height: 100px;
}
.container-3 .item {
width: 300px;
height: 100px;
position: absolute;
left: 50%;
margin-left: -150px;
}
</style>
</head>
<body>
<div class="container container-1">
<span>一段文字</span>
</div>
<div class="container container-2">
<div class="item">
this is block item
</div>
</div>
<div class="container container-3">
<div class="item">
this is absolute item
</div>
</div>
</body>
</html>
垂直居中
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>垂直对齐</title>
<style type="text/css">
.container {
border: 1px solid #ccc;
margin: 10px;
padding: 10px;
height: 200px;
}
.item {
background-color: #ccc;
}
.container-1{
text-align: center;
line-height: 200px;
height: 200px;
}
.container-2 {
position: relative;
}
.container-2 .item {
width: 300px;
height: 100px;
position: absolute;
left: 50%;
margin-left: -150px;
top: 50%;
margin-top: -50px;
}
.container-3 {
position: relative;
}
.container-3 .item {
width: 200px;
height: 80px;
position: absolute;
left: 50%;
top: 50%;
transform: translate(-50%, -50%)
}
.container-4 {
position: relative;
}
.container-4 .item {
width: 100px;
height: 50px;
position: absolute;
top: 0;
left: 0;
bottom: 0;
right: 0;
margin: auto;
}
</style>
</head>
<body>
<div class="container container-1">
<span>一段文字</span>
</div>
<div class="container container-2">
<div class="item">
this is item
</div>
</div>
<div class="container container-3">
<div class="item">
this is item
</div>
</div>
<div class="container container-4">
<div class="item">
this is item
</div>
</div>
</body>
</html>