iphoneX 适配

1.iphoneX的尺寸  375*812   上边有44px的危险区，下边有34px的危险区，剩下的是安全区。

2.viewport设值cover

 <meta name="viewport" content="width=device-width,initial-scale=1.0, minimum-scale=1.0, maximum-scale=1.0, user-scalable=no,viewport-fit=cover">

3.媒体查询适配样式

 @media only screen and (device-width: 375px) and (device-height: 812px) and (-webkit-device-pixel-ratio: 3) {

 }