code: function on (el, event, handler) {
    if (el.addEventListener) {
      el.addEventListener(event, handler)
    } else {
      el.attachEvent('on' + event, function () {
        handler.call(el)
      })
    }
  }
docs: on : on(el, event, fn)
  Adds an event handler.
  
  var on = require('dom101/on');
  
  on(el, 'click', function () {
  ...
  });