关于 iphone刘海屏适配

默认刘海屏的webview是有安全区域的，加上viewport-fit=cover属性后webview会全屏，全屏后为了适配刘海屏加上安全区域

padding-top: 1rem;//android样式

padding: env(safe-area-inset-top) env(safe-area-inset-right) env(safe-area-inset-bottom) env(safe-area-inset-left);// ios 样式

padding: constant(safe-area-inset-top) constant(safe-area-inset-right) constant(safe-area-inset-bottom) constant(safe-area-inset-left); // ios样式

ios两个样式原因
when the iPhone X first came out, it used  constant() instead of  env(), but as of 11.2,  constant() has been removed in favor of the standardized  env()