tabbing.js

(function (window, document) {
  'use strict';

  let className = 'tabbing';

  let handleFirstTab = function (e) {
    if (e.keyCode === 9) {
      document.body.classList.add(className);
      window.removeEventListener('keydown', handleFirstTab);
      window.addEventListener('mousedown', handleMouseDownOnce);
    }
  }

  let handleMouseDownOnce = function () {
    document.body.classList.remove(className);
    window.removeEventListener('mousedown', handleMouseDownOnce);
    window.addEventListener('keydown', handleFirstTab);
  }

  window.addEventListener('keydown', handleFirstTab);

})(window, document);