宽窄屏图片

有时候我们需要根据屏幕的款在，分辨率的不同设置两种大小的图片：

#test {
  background-image: url(assets/no-image-set.png); 
  background-image: -webkit-image-set(url(assets/test.png) 1x,url(assets/test-hires.png) 2x);
  background-image: -moz-image-set(url(assets/test.png) 1x,url(assets/test-hires.png) 2x);
  background-image: -o-image-set(url(assets/test.png) 1x,url(assets/test-hires.png) 2x);
  background-image: -ms-image-set(url(assets/test.png) 1x,url(assets/test-hires.png) 2x);
  width:200px;
  height:75px;
}