Basic MutationObserver usage.

mutationsnippet.js

  var mutationObserver = new MutationObserver(
    function respondToMutation(mutations, observer) {
      if (mutations.length < 1 || mutations[0].addedNodes.length !== 1) {
        alert('Could not load a player to play the videos. Sorry.');
      }
      else {
        // Do a thing!
      }
    }
  );

  mutationObserver.observe(playerContainerEl, {
    attributes: true, childList: true, characterData: true
  });