h5 寬度全屏自適應

 <style>
        html{
            font-size:calc(100vw/640*100); 
        }
        .main{
            position: absolute;
            top: 0;
            left: 0;
            width: 6.4rem;
            height: 6.4rem;
            background:red;
        }
    </style>