最新写法：CSS3，不兼容IE8-
position: fixed;top: 50%;left: 50%;width: 50%;max-width: 630px;min-width: 320px;height: auto;z-index: 2000;visibility: hidden;-webkit-backface-visibility: hidden;-moz-backface-visibility: hidden;backface-visibility: hidden;-webkit-transform: translateX(-50%) translateY(-50%);-moz-transform: translateX(-50%) translateY(-50%);-ms-transform: translateX(-50%) translateY(-50%);transform: translateX(-50%) translateY(-50%);CSS3 不定宽高水平垂直居中
justify-content:center;//子元素水平居中align-items:center;//子元素垂直居中display:-webkit-flex;