@media screen
参考地址:
http://www.swordair.com/blog/2010/08/431/
http://ashaochangfu.blog.163.com/blog/static/104251730201210245438273/
http://newhtml.net/通过-css-media-query-制作感知屏幕尺寸的表格/
http://webdesignerwall.com/tutorials/css3-media-queries
http://www.ruanyifeng.com/blog/2012/05/responsive_web_design.html