Encapsulates this hack from before MutationObservers existed (IE10+, no-ops on older browsers, MIT license): https://davidwalsh.name/detect-node-insertion

nodeInsertion.js

var nodeInsertion = (function (document, undefined) {
'use strict';
function noop() {}
var result = {add: noop, remove: noop};
if (typeof String.prototype.trim !== 'function') return result; // Every browser that supports animations also has String#trim

// a stylesheet that holds our special keyframe code: https://davidwalsh.name/add-rules-stylesheets
var animName = ('node_' + Math.random() + '_inserted').split('.').join(''), sheet = (function () {
    var style = document.createElement('style');
    style.appendChild(document.createTextNode(['@-o-','@-moz-', '@-webkit-', '@', ''].join('keyframes ' +
      animName + ' {\n  from {opacity: 0.99;}\n  to {opacity: 1;}\n}\n')));
    document.head.appendChild(style);
    return style.sheet;
  })(), addCSSRule = function (sheet, selector, rules, index) {
    if (typeof sheet.insertRule === 'function') addCSSRule = function addCSSRule(sheet, selector, rules, index) {
      return sheet.insertRule(selector + '{' + rules + '}', index);
    };
    else if (typeof sheet.addRule === 'function') addCSSRule = function addCSSRule(sheet, selector, rules, index) {
      return sheet.addRule(selector, rules, index);
    };
    else addCSSRule = noop;
    return addCSSRule(sheet, selector, rules, index);
  }, animEvents = ['oAnimationStart','mozAnimationstart', 'MSAnimationStart', 'webkitAnimationStart', 'animationstart'],
  alen = animEvents.length, prefixes = ['-o-', '-moz-', '-webkit-', '', ''], canDelete = typeof sheet.deleteRule === 'function',
  animEvent = null, animFound = false, listeners = [], insertionAnim = prefixes.join('animation-duration: 0.001s; ') +
    prefixes.join('animation-name: ' + animName + '; ');

if (addCSSRule === noop) return result;

var checkRule = function checkRule(rules, selector, index) {
    return index in rules && rules[index].cssText.split('{')[0].indexOf(selector) !== -1;
  }, defaultSel = function defaultSel(sel, def) {
    return typeof sel !== 'string' ? def : sel.trim() || def;
  };

// listens solely for new direct descendants
// returns index to be passed into the `remove` method
result.add = function add(func, sel, desc) {
  if (typeof func !== 'function') throw new TypeError('A function must be passed in');
  var s = defaultSel(sel, 'body'), d = defaultSel(desc, '*'), elts = document.querySelectorAll(s), i = elts.length,
    rules = sheet.cssRules, len = rules.length, selector = s + ' > ' + d, wrapped = function wrapped(e) {
      var evt = e || event;
      if (!animEvent) animEvent = evt.type;
      if (evt.type === animEvent && evt.animationName === animName) return func.call(this, evt);
    }, j;
  listeners.push({func: wrapped, sel: s, desc: d});
  while (i--) {
    if (animEvent) elts[i].addEventListener(animEvent, wrapped, false);
    else for (j = alen; j--;) elts[i].addEventListener(animEvents[j], wrapped, false);
  }
  for (i = len; i--;) if (checkRule(rules, selector, i)) return listeners.length;
  addCSSRule(sheet, selector, insertionAnim, len);
  return listeners.length;
};

result.remove = function remove(index) {
  if (!((index - 1) in listeners)) return;
  var listener = listeners[index - 1], func = listener.func, s = listener.sel,
    elts = document.querySelectorAll(s), i = elts.length, j;
  while (i--) {
    if (animEvent && animFound) elts[i].removeEventListener(animEvent, func, false);
    else {
      if (animEvent) animFound = true;
      for (j = alen; j--;) elts[i].removeEventListener(animEvents[j], func, false);
    }
  }
  if (canDelete) {
    var rules = sheet.cssRules, selector = s + ' > ' +  listener.desc;
    for (i = rules.length; i--;) if (checkRule(rules, selector, i)) sheet.deleteRule(i);
  }
  listeners.splice(index - 1);
};
return result;
})(document);