div上下左右都居中对齐，要求ie ff gg浏览器都支持
http://www.jb51.net/css/28259.html
<!DOCTYPE html>
盒子水平垂直都居中
*{ padding: 0; margin: 0; } .box{ width: 200px; height: 200px; background: red; position: absolute; left: 50%; top: 50%; margin-top: -100px; margin-left: -100px; }