HTML：
<body> <div class="maxbox"> <div class="minbox align-center"></div> </div></body>
父元素
.maxbox{ position: relative; width: 500px; height: 500px; margin: 5px; box-shadow: 1px 1px 1px rgba(0, 0, 0, 0.8), -1px -1px 1px rgba(0, 0, 0, 0.8);} 子元素：
.minbox{ width: 200px; height: 200px; box-shadow: 1px 1px 1px rgba(0, 0, 0, 0.8), -1px -1px 1px rgba(0, 0, 0, 0.8);} 效果图（下面几种方法效果图一样）：
第一种： CSS绝对定位
主要利用绝对定位，再用margin设置为auto
水平垂直居中对齐：
.align-center{ position: absolute; top: 0; left: 0; bottom: 0; right: 0; margin: auto;} 第二种： CSS绝对定位
主要利用绝对定位，再用margin调整到中间位置。
水平垂直居中对齐：
.align-center{ position: absolute; left: 50%; top: 50%; margin-left: -100px; /*width/-2*/ margin-top: -100px; /*height/-2*/} 第三种： CSS绝对定位 + Javascript/JQuery
主要利用绝对定位，再用Javascript/JQuery调整到中间位置。相比第一种方法，此方法提高了class的灵活性。
水平垂直居中对齐：
.align-center{ position: absolute; left: 50%; top: 50%;} JQuery：
$(function(){ $(".align-center").css( { "margin-left": ($(".align-center").width()/-2), "margin-top": ($(".align-center").height()/-2) } );}); 第四种： CSS3绝对定位 + 位移
使用绝对定位与CSS3的 transform: translate同样也可以达到效果。
水平垂直居中对齐：
.align-center{ position: absolute; top: 50%; left: 50%; -webkit-transform: translate(-50%, -50%); -moz-transform: translate(-50%, -50%); transform: translate(-50%, -50%); /*向左向上位移*/} 第五种： Flexbox: [伸缩布局盒模型]
要让元素水平垂直，对于Flexbox模型来说太容易了。
这里得改变一下HTML：
<div class="maxbox align-center"> <div class="minbox"></div></div>
水平垂直居中对齐：
.align-center{ display: flex; display: -webkit-flex; /*兼容问题*/ justify-content: center; align-items: center; } 几种方法的比较：
第一种：使用CSS绝对定位margin设置为auto，兼容性很好，应该是一种最好的方法了。
第二种：使用CSS绝对定位margin调整，兼容性很好但是欠缺灵活性。如果有很多box里需要水平垂直居中，因其width，height不同而需要写不同的 .align-center 。
第三种：使用脚本语言，兼容性很好且弥补了第一种的缺点。不因width，height的改变而影响水平垂直居中的效果。
第四种：使用CSS3的一些新的属性，兼容IE10, Chrome, Firefox, 和 Opera。兼容性不太很好，不因width，height的改变而影响水平垂直居中的效果。
第五种：使用Flexbox模型，兼容Firefox、Opera 和 Chrome，IE全军覆没。也是不因width，height的改变而影响水平垂直居中的效果。
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