(function (doc, win) {

    var docEl = doc.documentElement,

        resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',

        recalc = function () {

            var clientWidth = docEl.clientWidth;

            if (!clientWidth) return;

            if(clientWidth>=640){

                docEl.style.fontSize = '100px';

            }else{

                docEl.style.fontSize = 100 * (clientWidth / 640) + 'px';

            }

        };

    if (!doc.addEventListener) return;

    win.addEventListener(resizeEvt, recalc, false);

    doc.addEventListener('DOMContentLoaded', recalc, false);

})(document, window);