(function(doc, win) {
  let docEl = doc.documentElement,
    resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
    recalc = function() {
      let clientWidth = docEl.clientWidth;
      if (!clientWidth) return;
      if (clientWidth < 1000) clientWidth = 1000;
      docEl.style.fontSize = (clientWidth / 19.2 / 2) + 'px';
    };
  if (!doc.addEventListener) return;
  recalc();
  win.addEventListener(resizeEvt, recalc, false);
  doc.addEventListener('DOMContentLoaded', recalc, false);
})(document, window);