margin负值

http://www.cnblogs.com/2050/archive/2012/08/13/2636467.html#2457812

http://www.bbs0101.com/archives/34.html

http://www.bbs0101.com/archives/42.html

http://www.cnblogs.com/jscode/archive/2012/08/28/2660078.html

http://www.hicss.net/i-know-you-do-not-know-the-negative-margin/