1. 水平垂直居中
(1). inline
* 内容为文本、图片
* text-align:center; vertical-align:middle;
* display:table-cell; text-align:center; vetical-align:middle;
(2). block
2.1 水平垂直
* .content{ position: relative;}
.wrap{ position:absoulute; left:50%; margin-left:-20px; width:40px; height:40px;}
* .content { position: relative;}
.wrap { margin: auto; position: absolute; left:0; right:0; top:0 ; bottom:0;}
* .content{ display:table;}
.wrap { display:table-cell; text-align:center; vetical-align:middle;}
* .content{ display:flex; justify-content:center; align-items:center;}
2.2 水平居中 .content>.wrap
* .wrap{ margin: 0 auto;}
* 浮动元素的水平居中
>> 确定浮动元素的宽
.content > .f-width > .wrap
.f-width{ position:relative; left: 50%;}
.wrap { position:relative; left: -50%;}
*****
.wrap 必须用position:relative;