alternate: 替換
1. <link rel="alternate" media="only screen and (max-width: 640px)" href=" http://m.home.com/" />
解釋：當屏幕寬度小於640px時，頁面跳轉到 http://m.home.com/
2. <lik rel="stylesheet" media="screen" href="http://www.home.com/screen.css" /> 首選樣式
<link rel="alternate stylesheet" media="print" href="http://www.home.com/print.css" /> 替換樣式