css

 (function(doc,win){
    let docEL = doc.documentElement
    let resize = 'orientationChange' in window?'orientationChange':'resize'
    let resizeRem = (){
        docEL.style.fontSize = 20*docEL.clientWidth/640+'px'
    }
    win.addEventListener(resize,resizeRem,false)
})(document,window)