rem1

<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width,initial-scale=1,minimum-scale=1,maximum-scale=1,user-scalable=no" />

        <title></title>
        <style type="text/css">

            *{
                margin: 0;
                padding: 0;
                box-sizing: border-box;
            }
            #div1{
                width: 3.75rem;
                height: 2rem;
                background: skyblue;
                font-size: 2rem;
            }
        </style>
    </head>
    <body>
        <div id='div1'>
            ccccc
        </div>
        <script type="text/javascript">
            var html=document.querySelector('html')
            html.style.fontSize=window.innerWidth/7.5 +'px'
            window.onresize=function(e){
                html.style.fontSize=window.innerWidth/7.5 +'px'
            }
        </script>
    </body>
</html>