实现css水平垂直居中的方法有很多，在这里我简单的说下四种比较常用的方法：
1.使用CSS3中的Flex布局
对于flex，我们要了解的是它是一个display的属性，而且必须要给他的父元素设置flex属性(flex必须配合绝对定位使用！！！！！)，除了设置display：flex之外，还有另外两个属性需要设置，分别是justify-content和align-items，他们的意思分别是水平居中和垂直居中。HTML+CSS代码如下：
body { position: absolute; width: 100%; height: 100%; display: flex; justify-content: center; align-items: center;}.parentNode { width: 400px; height: 400px; background: #f00;}
<body> <div class="parentNode"></div></body>
当需求改变时，比如我们要在此div里面嵌套一个div，根据我上面提到的，要想子DIV垂直水平居中，我们也要给父DIV同样这样设置
<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>Document</title> <style> * { margin: 0; padding: 0; } body { position: absolute; // flex必须配合absolute使用才会生效 width: 100%; height: 100%; display: flex; justify-content: center; align-items: center; } .parentNode { width: 400px; height: 400px; background: #f00; position: relative; // 这里必须用relative 原因是 相对于 body这个父标签定位 如果用absolute会找上级的relative，如果没有，就到顶级的document display: flex; justify-content: center; align-items: center; } .childNode { width: 200px; height: 200px; background: #fff; } </style></head><body> <div class="parentNode"> <div class="childNode"></div> </div></body></html> 2.使用CSS3中的transform
.parentNode { width: 400px; height: 400px; background: #f00; position: absolute; left: 50%; top: 50%; transform: translate(-50%, -50%);}
3.当你知道元素的width与height时，使用CSS2中的最普通不过的margin
.parentNode { width: 400px; height: 400px; background: #f00; position: absolute; left: 50%; top: 50%; margin: -200px 0 0 -200px;}
4.使用比较特殊的margin：auto
.parentNode { width: 400px; height: 400px; background: #f00; overflow: auto; margin: auto; // 在标准流的情况下，让 margin-top 以及 margin-bottom 都为0 position: absolute; top: 0; left: 0; bottom: 0; right: 0; // 使浏览器对其元素所在的区域内重新渲染，四个值都设为0目的是让整个窗口都为该元素的重新渲染区域，之后margin-top以及margin-bottom都相等 } 现在让我们来使用强大的SASS重构一下这几个样式，先拿flex开刀吧，
@mixin center { display: flex; justify-content: center; align-items: center;}body { position: absolute; width: 100%; height: 100%; @include center;<br /> .parentNode { width: 400px; height: 400px; background: #f00; position: relative; @include center;<br /> .childNode { width: 200px; height: 200px; background: #fff; } }} 如果你的整个网站中有几处小样式类似，比如颜色，字体等，在 Sass 可以使用"$"变量来统一处理，那么这种选择还是不错的。但当你的样式变得越来越复杂，需要重复使用大段的样式时，使用变量就无法达到我们目了。这个时候 Sass 中的混合宏就会变得非常有意义,@mixin 是用来声明混合宏的关键词，有点类似 CSS 中的 @media、@font-face 一样。center 是混合宏的名称。大括号里面是复用的样式代码。@include为调用混合宏。除了声明一个不带参数的混合宏之外，还可以在定义混合宏时带有参数，并且在里面还可以写更加复杂的逻辑。
下面我将会用到if else语句以及@mixin混合宏来封装我们上面的第2,3,4方法。
我们的思路是先将DIV的左上角绝对定位到容器的中心位置，然后为 mixin 添加两个可选参数($width,$height)，分别代表元素的宽高，如果传递了参数，那么就使用负向 margin 的方法实现居中；如果没有传递参数，就使用 CSS3的transform 的方法。
/** * 为子元素设定定位上下文 */.parent { position: relative;}/** * 让子元素绝对居中于父容器 * 没有向 Sass mixin 传递宽和高，使用 CSS transform 属性实现居中效果 */.child-with-unknown-direction { @include center;}/** * 让子元素绝对居中于父容器 * 向 Sass mixin 传递了宽度，所以就使用负向 margin 处理水平位置， * 使用 CSS transform translateY 处理垂直位置 */.child-with-known-width { @include center(400px);}/** * 让子元素绝对居中于父容器 * 向 Sass mixin 传递了高度，所以就使用负向 margin 处理垂直位置， * 使用 CSS transform translateX 处理水平位置 */.child-with-known-height { @include center($height: 400px);}/** * 让子元素绝对居中于父容器 * 向 Sass mixin 传递了高度和宽度，所以就使用负向 margin 处理水平和垂直位置 */.child-with-known-direction { @include center(400px, 400px);} 现在我们开始封装@mixin，由上面的CSS分析知，要实现居中必须先让元素绝对定位
@mixin center($width: null, $height: null) { position: absolute; top: 50%; left: 50%;} 然后根据下面的逻辑搭建@mixin的骨架
| null | null | translate |
| defined | defined | margin |
| defined | null | margin-left + translateY |
| null | defined | margin-right + translateX |
@mixin center($width:null,$height:null){ display: flex; justify-content: center; align-items: center; @if $width and $height { // do margin } @else if not $width and not $height { // do transform translate(-50%,-50%) } @else if not $width { // do margin-top and transform translateX } @else { // do margin-left and transform translateY }} 最后我们把具体的代码插入到不同的条件中去
@mixin center($width:null,$height:null){ position: absolute; top: 50%; left: 50%; @if $width and $height { // do margin width: $width; height: $height; margin: -($height / 2) #{0 0} -($width / 2); //这里如果直接写 0 0 他会编译为 margin: xx 0 xx 而不是 margin：xx 0 0 xx，所以用 #{0 0}<br /> } @else if not $width and not $height { // do transform translate(-50%,-50%) transform: translate(-50%,-50); } @else if not $width { // do margin-top and transform translateX height: $height; margin-top: -(height / 2); transform: translateX(-50%); } @else { // do margin-left and transform translateY width: $width; margin-top: -(width / 2); transform: translateY(-50%); }} 最后我们可以通过Koala软件离线编译也可以通过http://www.sassmeister.com/在线编译，下面是编译好的结果
@charset "UTF-8";/** * 为子元素设定定位上下文 */.parent { position: relative;}/** * 让子元素绝对居中于父容器 * 没有向 Sass mixin 传递宽和高，使用 CSS transform 属性实现居中效果 */.child-with-unknown-direction { position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50);}/** * 让子元素绝对居中于父容器 * 向 Sass mixin 传递了宽度，所以就使用负向 margin 处理水平位置， * 使用 CSS transform translateY 处理垂直位置 */.child-with-known-width { position: absolute; top: 50%; left: 50%; width: 400px; margin-top: -width/2; transform: translateY(-50%);}/** * 让子元素绝对居中于父容器 * 向 Sass mixin 传递了高度，所以就使用负向 margin 处理垂直位置， * 使用 CSS transform translateX 处理水平位置 */.child-with-known-height { position: absolute; top: 50%; left: 50%; height: 400px; margin-top: -height/2; transform: translateX(-50%);}/** * 让子元素绝对居中于父容器 * 向 Sass mixin 传递了高度和宽度，所以就使用负向 margin 处理水平和垂直位置 */.child-with-known-direction { position: absolute; top: 50%; left: 50%; width: 400px; height: 400px; margin: -200px 0 0 -200px;}
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