1. CSS浮动(float,clear)通俗讲解:http://www.cnblogs.com/iyangyuan/archive/2013/03/27/2983813.html
http://www.cnblogs.com/bokin/archive/2012/12/14/2816864.html
http://www.cnblogs.com/polk6/archive/2013/07/26/3214847.html
http://blog.jobbole.com/49320/
3. z-index: http://www.cnblogs.com/ForEvErNoME/p/3373641.html