meta [设置全屏]
<meta name="viewport" content="width=device-width, initial-scale=1.0,viewport-fit=cover"
viewport-fit 的值有 默认值-auto/contain，全屏值-cover
constant() 以及 safe-area-inset-top safe-area-inset-right safe-area-inset-bottom safe-area-inset-left 是 iOS 11 webview 新增加的特性，专门用于应对刘海
body {
padding: constant( safe-area-inset-top ) constant( safe-area-inset-right ) constant( safe-area-inset-bottom) constant( safe-area-inset-left)
}
//默认值
safe-area-inset-top=0， safe-area-inset-right=44px ， safe-area-inset-bottom=21px， safe-area-inset-left=44px
