css3实现垂直居中-flex

关键兼容代码：

  display: -webkit-box;  display: -webkit-flex;    display: -moz-box;  display: -moz-flex;    display: -ms-flexbox;    display: flex;    /* 水平居中*/  -webkit-box-align: center;  -moz-box-align: center;  -ms-flex-pack:center;/* IE 10 */    -webkit-justify-content: center;  -moz-justify-content: center;  justify-content: center;/* IE 11+,Firefox 22+,Chrome 29+,Opera 12.1*/    /* 垂直居中 */  -webkit-box-pack: center;  -moz-box-pack: center;  -ms-flex-align:center;/* IE 10 */    -webkit-align-items: center;  -moz-align-items: center;  align-items: center;