这个答案提供了一种方便的方法来生成iOS目前所需的所有20个启动画面 + 最新的HTML标记以支持iOS 12.1。
其中包括iPhone XR、iPhone XS Max和11英寸iPad Pro的解决方案。
背景
iOS上的Safari现在支持渐进式Web应用程序，但其实现方式与Chrome不同。它确实读取manifest文件，但忽略其中声明的图标。
相反，Safari期望一个apple-touch-startup-image标签列表。然而，官方的苹果文档列出了以下示例：
<link rel="apple-touch-startup-image" href="/launch.png">
在iOS 12.1及以后的版本中，只有一个图片不足以满足要求，这会导致误导。相反，Safari期望每个分辨率都有一个图片。
如果启动画面缺失或不正确，加载时将显示一个白屏，这看起来不专业，使（Web）应用程序感觉缓慢。
生成启动画面
在线上有一些apple-touch-startup-image生成器，但它们可能不能正常工作，或者完全忽略了Landscape模式，并且它们的命名规则也不是那么友好。这种方式更加简便。
在包含logo.png文件的目录中，以bash运行以下命令，它将生成Safari所需的20个图像（每个纵向和横向各10个分辨率）：
array=( 0640x1136 0750x1334 0828x1792 1125x2436 1242x2208 1242x2688 1536x2048 1668x2224 1668x2388 2048x2732 )
for i in "${array[@]}"
do
split=(${i//x/ })
portrait=$i
landscape=${split[1]}x${split[0]}
gm convert -background white -geometry $((10#${split[0]} / 5)) "logo.png" -gravity center -extent ${portrait} splash-portrait-${portrait}.png
gm convert -background white -geometry $((10#${split[0]} / 5)) "logo.png" -gravity center -extent ${landscape} splash-landscape-${landscape}.png
done
这依赖于 GraphicsMagick，它是 ImageMagick 的更好替代品。在 macOS 上，使用 brew 安装非常容易，只需运行 brew install graphicsmagick。
标记
以下是所有 20 个文件的 HTML 标记：
<!--
# SPLASH SCREENS FOR iOS.
#
# If the splash screen is missing or incorrect, a white screen will show on load, which looks unprofessional
# and makes the (web)app feel slow.
#
# Constraints:
# - Cannot use a single image for all.
# - The size of the image must fit that of the targeted device, else no splash screen will show.
# - There seems to be some leeway (e.g.: pictures 40px and 60px shorter did work), but unclear how much.
# Bottom-line: you need one splash screen per resolution and orientation.
#
#
# List of devices and resolutions (AUG-2019):
#
# Device Portrait size Landscape size Screen size Pixel ratio
# iPhone SE 640px × 1136px 1136px × 640px 320px × 568px 2
# iPhone 8 750px × 1334px 1334px × 750px 375px × 667px 2
# iPhone 7 750px × 1334px 1334px × 750px 375px × 667px 2
# iPhone 6s 750px × 1334px 1334px × 750px 375px × 667px 2
# iPhone XR 828px × 1792px 1792px × 828px 414px × 896px 2
# iPhone XS 1125px × 2436px 2436px × 1125px 375px × 812px 3
# iPhone X 1125px × 2436px 2436px × 1125px 375px × 812px 3
# iPhone 8 Plus 1242px × 2208px 2208px × 1242px 414px × 736px 3
# iPhone 7 Plus 1242px × 2208px 2208px × 1242px 414px × 736px 3
# iPhone 6s Plus 1242px × 2208px 2208px × 1242px 414px × 736px 3
# iPhone XS Max 1242px × 2688px 2688px × 1242px 414px × 896px 3
# 9.7" iPad 1536px × 2048px 2048px × 1536px 768px × 1024px 2
# 7.9" iPad mini 4 1536px × 2048px 2048px × 1536px 768px × 1024px 2
# 10.5" iPad Pro 1668px × 2224px 2224px × 1668px 834px × 1112px 2
# 11" iPad Pro 1668px × 2388px 2388px × 1668px 834px × 1194px 2
# 12.9" iPad Pro 2048px × 2732px 2732px × 2048px 1024px × 1366px 2
#
# Sources:
# - Device and resolutions (Portrait size, Landscape size) from https://developer.apple.com/design/human-interface-guidelines/ios/icons-and-images/launch-screen/
# - Screen width as measured by JavaScript's `screen.width` and `screen.height` in Simulator, except for:
# - 7.9" iPad mini 4 (not in Simulator): https://developer.apple.com/library/archive/documentation/DeviceInformation/Reference/iOSDeviceCompatibility/Displays/Displays.html
# - 9.7" iPad (not in Simulator): had to assume they meant iPad Pro.
#
#
# Tested on the following devices, in Simulator with iOS 12.1, in both Portrait and Landscape:
# iPhone 5s, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPhone 8,
# iPhone 8 Plus, iPhone SE, iPhone X, iPhone XS, iPhone XS Max, iPhone XR, iPad Air, iPad Air 2,
# iPad (5th generation), iPad Pro (9.7-inch), iPad Pro (12.9-inch), iPad Pro (12.9-inch) (2nd generation),
# iPad Pro (10.5-inch), iPad (6th generation), iPad Pro (11-inch), iPad Pro (12.9-inch) (3rd generation).
# Everything worked fine (splash screen showing in every single case.)
#
# FYI:
# - tvOS does not come with a browser. So the largest screen to care for is an iPad.)
# - On all iPads (in Simulator), had to either Add to Home twice or restart the device to see the icon.
# WOULDDO Test on a real iPad.
-->
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-0640x1136.png" media="(device-width: 320px) and (device-height: 568px) and (-webkit-device-pixel-ratio: 2) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-0750x1334.png" media="(device-width: 375px) and (device-height: 667px) and (-webkit-device-pixel-ratio: 2) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-0828x1792.png" media="(device-width: 414px) and (device-height: 896px) and (-webkit-device-pixel-ratio: 2) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-1125x2436.png" media="(device-width: 375px) and (device-height: 812px) and (-webkit-device-pixel-ratio: 3) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-1242x2208.png" media="(device-width: 414px) and (device-height: 736px) and (-webkit-device-pixel-ratio: 3) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-1242x2688.png" media="(device-width: 414px) and (device-height: 896px) and (-webkit-device-pixel-ratio: 3) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-1536x2048.png" media="(device-width: 768px) and (device-height: 1024px) and (-webkit-device-pixel-ratio: 2) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-1668x2224.png" media="(device-width: 834px) and (device-height: 1112px) and (-webkit-device-pixel-ratio: 2) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-1668x2388.png" media="(device-width: 834px) and (device-height: 1194px) and (-webkit-device-pixel-ratio: 2) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-portrait-2048x2732.png" media="(device-width: 1024px) and (device-height: 1366px) and (-webkit-device-pixel-ratio: 2) and (orientation: portrait)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-1136x0640.png" media="(device-width: 320px) and (device-height: 568px) and (-webkit-device-pixel-ratio: 2) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-1334x0750.png" media="(device-width: 375px) and (device-height: 667px) and (-webkit-device-pixel-ratio: 2) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-1792x0828.png" media="(device-width: 414px) and (device-height: 896px) and (-webkit-device-pixel-ratio: 2) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-2436x1125.png" media="(device-width: 375px) and (device-height: 812px) and (-webkit-device-pixel-ratio: 3) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-2208x1242.png" media="(device-width: 414px) and (device-height: 736px) and (-webkit-device-pixel-ratio: 3) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-2688x1242.png" media="(device-width: 414px) and (device-height: 896px) and (-webkit-device-pixel-ratio: 3) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-2048x1536.png" media="(device-width: 768px) and (device-height: 1024px) and (-webkit-device-pixel-ratio: 2) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-2224x1668.png" media="(device-width: 834px) and (device-height: 1112px) and (-webkit-device-pixel-ratio: 2) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-2388x1668.png" media="(device-width: 834px) and (device-height: 1194px) and (-webkit-device-pixel-ratio: 2) and (orientation: landscape)">
<link rel="apple-touch-startup-image" href="/assets/site/img/splash/splash-landscape-2732x2048.png" media="(device-width: 1024px) and (device-height: 1366px) and (-webkit-device-pixel-ratio: 2) and (orientation: landscape)">
(个人而言，我会将注释放在Twig注释块中，这样就可以保留信息而不会使客户端太过冗长。)
在线上看到的一些示例使用了min-device-*，但在这种情况下，这是没有多大意义的，因为Safari期望图片具有（近似）精确的尺寸。
我看到的其他一些示例使用较短的图像（比如少40或60像素状态栏的图像）。早期版本的iOS似乎期望这种尺寸，但现在不再是这样了。
结束语
我的96% iOS用户使用iOS 12.x，所以我们不需要太在意较老的iOS版本。但如果我漏掉了什么，请让我知道。
就像一个大男孩一样，Android很高兴将应用程序的图标作为启动画面的一部分显示出来，而iOS和Safari则迫使我们进行所有这些额外的工作。要显示一个简单的启动画面需要20张图片！这太疯狂了！事情可能会变得更好，但现在就是这样。这项基本任务需要大量编码和测试。我希望它能帮助到某些人。我将尝试更新较新的分辨率。如果您发现缺少，请在评论中留言。
