减少图片能减少HTTP请求,同时减少页面大小,更容易维护,常用的属性如下: * linear and radial gradients * border-radius * box-shadow * rgba * transform * css mask 常见的使用场景有: [polished buttons via gradients](http://cubiq.org/dropbox/cssgrad.html), [replicate many other effects](http://www.phpied.com/css-performance-ui-with-fewer-images/)