//This is a complete guide for centering a div with CSS.
//见过许多方法来居中div，当然有些方法因情况不同而不同
//详细可参考：
//1.Centering in CSS: A Complete Guide（https://css-tricks.com/centering-css-complete-guide/）
//2.How To Center Anything With CSS（https://codemyviews.com/blog/how-to-center-anything-with-css）
//3.CSS layout: tricks and layout techniques in ‘Learn CSS Layout’（http://book.mixu.net/css/5-tricks.html）
//4.Absolute Centering（http://codepen.io/shshaw/full/gEiDt）