alternate: 替换
1. <link rel="alternate" media="only screen and (max-width: 640px)" href=" http://m.home.com/" />
解释：当屏幕宽度小于640px时，页面跳转到 http://m.home.com/
2. <lik rel="stylesheet" media="screen" href="http://www.home.com/screen.css" /> 首选样式
<link rel="alternate stylesheet" media="print" href="http://www.home.com/print.css" /> 替换样式