index.js

(function (doc, win) {
  var docEl = doc.documentElement,
  resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
  recalc = function () {
    var clientWidth = docEl.clientWidth < 680 ? docEl.clientWidth : 680;
    if (!clientWidth) return;
    docEl.style.fontSize = 100 * (clientWidth / 414) + 'px';
  };
  if (!doc.addEventListener) return;
  win.addEventListener(resizeEvt, recalc, false);
  doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);