判断横竖屏

@media (orientation: portrait) {   
    //竖屏  
}  
@media (orientation: landscape) {   
    //横屏  
}  

如：

@media screen and (orientation:landscape) {
    .cover {
        display: block;
    }
}

通过window.orientation来判断设备横竖屏

function checkOrient() {
        
    if (window.orientation == 0 || window.orientation == 180){
        var screenOrientation = 'portrait';
　　　　// 竖屏
    }
    else if (window.orientation == 90 || window.orientation == -90){
        var screenOrientation = 'landscape';
　　　　// 横屏
    }
    return screenOrientation;
}
// 添加事件监听
addEventListener('load', function(){
    checkOrient();
    window.onorientationchange = checkOrient;
});