ES5 Element ready observer using a MutationObserver to detect, if an HTML element generated on runtime is ready to be accessed.

elementReadyObserver.js

function isElement(addedNode, selector, callback) {
  if (typeof addedNode.querySelector === 'function' && addedNode.querySelector(selector)) {
    callback();
  }
}

var elementReadyObserver = new MutationObserver(function (mutations) {
  for (var i=0, numMutations=mutations.length; i<numMutations; i++) {
    for (var j=0, numNodes=mutations[i].addedNodes.length; j<numNodes; j++) {
      isElement(mutations[i].addedNodes[j], '#YOUR-CSS-SELECTOR > .OF-THE-ELEMENT.TO-OBSERVE', function() {
        elementReadyObserver.disconnect();
        // ... and do what you want to do with the element you've been observing
      });
    }
  }
});
elementReadyObserver.observe(document.body, {attributes: true, subtree: true, childList: true});