由于iPad 的启动画面是不包括状态栏区域的。所以启动图片需要减去状态栏区域所对应的方向上的20px大小，相应地在retina设备上要减去40px的大小。
<link href="apple-touch-startup-image-320×460.png" media="(device-width: 320px)" rel="apple-touch-startup-image">
<link href="apple-touch-startup-image-640×960.png" media="(device-width: 320px) and (-webkit-device-pixel-ratio: 2)" rel="apple-touch-startup-image">
<link href="apple-touch-startup-image-768×1004.png" media="(device-width: 768px) and (orientation: portrait)" rel="apple-touch-startup-image">
<link href="apple-touch-startup-image-748×1024.png" media="(device-width: 768px) and (orientation: landscape)" rel="apple-touch-startup-image">
<link href="apple-touch-startup-image-1536×2008.png" media="(device-width: 1536px) and (orientation: portrait) and (-webkit-device-pixel-ratio: 2)" rel="apple-touch-startup-image">
<link href="apple-touch-startup-image-2048×1496.png" media="(device-width: 1536px) and (orientation: landscape) and (-webkit-device-pixel-ratio: 2)" rel="apple-touch-startup-image">
（landscape：横屏 | portrait：竖屏）