监听 viewport 变化

var documentElement = document.documentElement;
function callback() {
var clientWidth = documentElement.clientWidth;
// 屏幕宽度大于780，不在放大
clientWidth = clientWidth < 780 ? clientWidth : 780;
documentElement.style.fontSize = clientWidth / 10 + 'px';
}

    document.addEventListener('DOMContentLoaded', callback);
  //判断方向变化
    window.addEventListener('orientationchange' in window ? 'orientationchange' : 'resize', callback);

https://developer.mozilla.org/en-US/docs/Web/Events/orientationchange