本文翻译自 CSS Tricks 中关于居中的相关介绍。
1、水平居中
1.1 内联元素的水平居中（如链接或文本）
可以在一个块级别的父元素中水平地居中内联元素，只需：
.center-children { ?text-align: center;}注：这种写法适用于inline, inline-block, inline-table, inline-flex等等。
代码示例：
=> html代码
<header> ?This text is centered.</header><nav role=‘navigation‘> ?<a href="#0">One</a> ?<a href="#0">Two</a> ?<a href="#0">Three</a> ?<a href="#0">Four</a></nav> ?=> css代码
body { ?background: #f06d06;}header, nav { ?text-align: center; ?background: white; ?margin: 20px 0; ?padding: 10px;}nav a { ?text-decoration: none; ?background: #333; ?border-radius: 5px; ?color: white; ?padding: 3px 8px;}=> 效果图
1.2 块级元素水平居中
可以通过给它设置margin-left和margin-right为auto （并且该元素需有width属性，否则它将占满整行，无法居中）来居中一个块级元素。通常用这样的速记法来做：
.center-me { ?margin: 0 auto;}注：无论您所处的块级别元素的宽度是多少，还是父节点，这都是可行的。
注意：你不能浮动一个元素到水平居中的位置 ，不过有一个小技巧（本文不再详述，可点击链接自行查看）。
代码示例：
=> html代码
<main> ?<div class="center"> ???I‘m a block level element and am centered. ?</div></main>=> css代码
body { ?background: #f06d06;}main { ?background: white; ?margin: 20px 0; ?padding: 10px;}.center { ?margin: 0 auto; ?width: 200px; ?background: black; ?padding: 20px; ?color: white;}=> 效果图
1.3 两个及以上的多个块级元素水平居中
如果您有两个或多个块级别的元素，它们需要在一行中水平居中，那么最好将它们作为不同的display。比如设置其为display:inline-block。
代码示例1：
注：下面的示例中，inline-block-center的<main>标签中，是把父元素设置text-align:center，子元素设置display:inline-block；
flex-center的<main>标签中，使用了CSS3中新增的弹性盒子（附带MDN相关介绍）是把父元素设置display: flex;justify-content: center;
=>html代码
<main class="inline-block-center"> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. ?</div> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. I have more content in me than my siblings do. ?</div> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. ?</div></main><main class="flex-center"> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. ?</div> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. I have more content in me than my siblings do. ?</div> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. ?</div></main>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}main { ?background: white; ?margin: 20px 0; ?padding: 10px;}main div { ?background: black; ?color: white; ?padding: 15px; ?max-width: 125px; ?margin: 5px;}.inline-block-center { ?text-align: center;}.inline-block-center div { ?display: inline-block; ?text-align: left;}.flex-center { ?display: flex; ?justify-content: center;}=>效果图
代码示例2：
注：下面的例子中，表现的是你有多个块级元素堆叠在一起，在这种情况下，自动边距（即margin:0 auto）仍然很好。即多个块级元素不需要在一行显示，居中的方法仍使用margin:0 auto。
=>html代码
<main> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. ?</div> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. I have more content in me than my siblings do. ?</div> ?<div> ???I‘m an element that is block-like with my siblings and we‘re centered in a row. ?</div></main>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}main { ?background: white; ?margin: 20px 0; ?padding: 10px;}main div { ?background: black; ?margin: 0 auto; ?color: white; ?padding: 15px; ?margin: 5px auto;}main div:nth-child(1) { ?width: 200px;}main div:nth-child(2) { ?width: 400px;}main div:nth-child(3) { ?width: 125px;}=>效果图
2、垂直居中
2.1 内联元素垂直居中
2.1.1 单行内联元素垂直居中
代码示例1：
单行内联元素使用上下相同的padding造成垂直居中的效果
.link { ?padding-top: 30px; ?padding-bottom: 30px;}=>效果图
代码示例2：
当某些情况下，不适合使用padding填充时，可以让其文本的行高=父元素的高度达到垂直居中的效果。
=>html代码
<main> ?<div> ???I‘m a centered line. ?</div></main>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}main { ?background: white; ?margin: 20px 0; ?padding: 40px;}main div { ?background: black; ?color: white; ?height: 100px; ?line-height: 100px; ?padding: 20px; ?width: 50%; ?white-space: nowrap;}=>效果图
2.1.2 多行内联元素垂直居中
代码示例1：
使用表格布局居中（基本过时的用法）
注：下面的例子中，第一个效果图展示的是直接使用<table>标签（其<td>标签默认垂直居中），第二个效果图展示的是父元素<div>设置display:table，子元素<p>设置display: table-cell;vertical-align: middle;
=>html代码
<table> ?<tr> ???<td> ?????I‘m vertically centered multiple lines of text in a real table cell. ???</td> ?</tr></table><div class="center-table"> ?<p>I‘m vertically centered multiple lines of text in a CSS-created table layout.</p></div>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}table { ?background: white; ?width: 240px; ?border-collapse: separate; ?margin: 20px; ?height: 250px;}table td { ?background: black; ?color: white; ?padding: 20px; ?border: 10px solid white; ?/* default is vertical-align: middle; */}.center-table { ?display: table; ?height: 250px; ?background: white; ?width: 240px; ?margin: 20px;}.center-table p { ?display: table-cell; ?margin: 0; ?background: black; ?color: white; ?padding: 20px; ?border: 10px solid white; ?vertical-align: middle;}=>效果图
代码示例2：
使用flexbox来垂直居中。
精简CSS代码如下：
.flex-center-vertically { ?display: flex; ?justify-content: center; ?flex-direction: column; ?height: 400px;}具体示例：=>html代码
<div class="flex-center"> ?<p>I‘m vertically centered multiple lines of text in a flexbox container.</p></div>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}div { ?background: white; ?width: 240px; ?margin: 20px;}.flex-center { ?background: black; ?color: white; ?border: 10px solid white; ?display: flex; ?flex-direction: column; ?justify-content: center; ?height: 200px; ?resize: vertical; ?overflow: auto;}.flex-center p { ?margin: 0; ?padding: 20px;}=>效果图
代码示例3：
使用伪元素来垂直居中。
精简CSS代码如下：
.ghost-center { ?position: relative;}.ghost-center::before { ?content: " "; ?display: inline-block; ?height: 100%; ?width: 1%; ?vertical-align: middle;}.ghost-center p { ?display: inline-block; ?vertical-align: middle;}=>html代码
<div class="ghost-center"> ?<p>I‘m vertically centered multiple lines of text in a container. Centered with a ghost pseudo element</p></div>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}div { ?background: white; ?width: 240px; ?height: 200px; ?margin: 20px; ?color: white; ?resize: vertical; ?overflow: auto; ?padding: 20px;}.ghost-center { ?position: relative;}.ghost-center::before { ?content: " "; ?display: inline-block; ?height: 100%; ?width: 1%; ?vertical-align: middle;}.ghost-center p { ?display: inline-block; ?vertical-align: middle; ?width: 190px; ?margin: 0; ?padding: 20px; ?background: black;}=>效果图
2.2 块级元素垂直居中
不知道网页布局的高度是很常见的，原因有很多：如果宽度改变，文本流会改变高度。文本样式的差异可以改变高度。文本数量的差异可以改变高度。具有固定宽宽比的元素，如图像，可以在重置宽高时改变高度等等。
但是如果你已经知道高度，就可以这样：
2.2.1 块级元素的高度已知/确定
即设置子元素相对于父元素绝对定位，子元素设置top：50%，并设置margin-top为负的子元素自身高度的一半。
.parent { ?position: relative;}.child { ?position: absolute; ?top: 50%; ?height: 100px; ?margin-top: -50px; /* account for padding and border if not using box-sizing: border-box; */}代码示例：
注：
=>html代码
<main> ???<div> ????I‘m a block-level element with a fixed height, centered vertically within my parent. ?</div> ?</main>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}main { ?background: white; ?height: 300px; ?margin: 20px; ?width: 300px; ?position: relative; ?resize: vertical; ?overflow: auto;}main div { ?position: absolute; ?top: 50%; ?left: 20px; ?right: 20px; ?height: 100px; ?margin-top: -70px; ?background: black; ?color: white; ?padding: 20px;}=>效果图
2.2.2 块级元素的高度未知/不确定
它仍然可以通过将它的一半高度推到一半的高度来居中，即设置子元素transform: translateY(-50%); ，即：
.parent { ?position: relative;}.child { ?position: absolute; ?top: 50%; ?transform: translateY(-50%);}=>html代码
<main> ???<div> ????I‘m a block-level element with an unknown height, centered vertically within my parent. ?</div> ?</main>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}main { ?background: white; ?height: 300px; ?margin: 20px; ?width: 300px; ?position: relative; ?resize: vertical; ?overflow: auto;}main div { ?position: absolute; ?top: 50%; ?left: 20px; ?right: 20px; ?background: black; ?color: white; ?padding: 20px; ?transform: translateY(-50%); ?resize: vertical; ?overflow: auto;}=>效果图
2.2.3 使用display:flex布局
即：
.parent { ?display: flex; ?flex-direction: column; ?justify-content: center;}=>html代码
<main> ???<div> ????I‘m a block-level element with an unknown height, centered vertically within my parent. ?</div> ?</main>=>css代码
body { ?background: #f06d06; ?font-size: 80%;}main { ?background: white; ?height: 300px; ?width: 200px; ?padding: 20px; ?margin: 20px; ?display: flex; ?flex-direction: column; ?justify-content: center; ?resize: vertical; ?overflow: auto;}main div { ?background: black; ?color: white; ?padding: 20px; ?resize: vertical; ?overflow: auto;}=>效果图
3、水平居中+垂直居中
3.1 固定宽高的元素
使用负的margin等于宽度和高度的一半，在你完全定位50%/50%之后，它会以巨大的跨浏览器支持为中心，即：
.parent { ?position: relative;}.child { ?width: 300px; ?height: 100px; ?padding: 20px; ?position: absolute; ?top: 50%; ?left: 50%; ?margin: -70px 0 0 -170px;}代码示例：
=>html代码
<main> ???<div> ????I‘m a block-level element a fixed height and width, centered vertically within my parent. ?</div> ?</main>=>css代码
body { ?background: #f06d06; ?font-size: 80%; ?padding: 20px;}main { ?position: relative; ?background: white; ?height: 200px; ?width: 60%; ?margin: 0 auto; ?padding: 20px; ?resize: both; ?overflow: auto;}main div { ?background: black; ?color: white; ?width: 200px; ?height: 100px; ?margin: -70px 0 0 -120px; ?position: absolute; ?top: 50%; ?left: 50%; ?padding: 20px;}=>效果图
3.2 宽高未知的元素
如果你不知道宽度和高度，你可以使用transform属性和在两个方向上的负translate（它基于当前元素的宽度/高度）到中心，即：
.parent { ?position: relative;}.child { ?position: absolute; ?top: 50%; ?left: 50%; ?transform: translate(-50%, -50%);}代码示例：
=>html代码
<main> ???<div> ????I‘m a block-level element of an unknown height and width, centered vertically within my parent. ?</div> ?</main>=>css代码
body { ?background: #f06d06; ?font-size: 80%; ?padding: 20px;}main { ?position: relative; ?background: white; ?height: 200px; ?width: 60%; ?margin: 0 auto; ?padding: 20px; ?resize: both; ?overflow: auto;}main div { ?background: black; ?color: white; ?width: 50%; ?transform: translate(-50%, -50%); ?position: absolute; ?top: 50%; ?left: 50%; ?padding: 20px; ?resize: both; ?overflow: auto;}=>效果图
使用flexbox居中
要用flexbox向两个方向居中，你需要使用两个中心属性，即：
.parent { ?display: flex; ?justify-content: center; ?align-items: center;}=>html代码
```
I‘m a block-level-ish element of an unknown width and height, centered vertically within my parent.
=>css代码body {
background: #f06d06;
font-size: 80%;
padding: 20px;
}
main {
background: white;
height: 200px;
width: 60%;
margin: 0 auto;
padding: 20px;
display: flex;
justify-content: center;
align-items: center;
resize: both;
overflow: auto;
}
main div {
background: black;
color: white;
width: 50%;
padding: 20px;
resize: both;
overflow: auto;
}
=>效果图#### 3.3 使用grid居中这只是一个非常有效的小技巧（由Lance Janssen上传）=>html代码=>css代码.wrapper{
height:300px;
width:300px;
border:1px solid red;
display: grid;
}
span {
margin: auto;
}
```
=>效果图
2018-05-22 CSS如何居中
原文地址：https://www.cnblogs.com/EnochQin/p/9073165.html