苹果Retina屏模糊，新浪是这样做的

background-image: -webkit-image-set(url(http://i1.sinaimg.cn/dy/deco/2013/0329/logo/LOGO_1x.png) 1x,url(http://i2.sinaimg.cn/dy/deco/2013/0329/logo/LOGO_2x.png) 2x);