MutationObserver doesn't observer inner frame on `document.write` in IE11

index.html

<h1>Run A Web Server Here Via <pre style="display: inline-block; background-color: #ddd; padding: 0.5em; border-radius: 5px;">python -m SimpleHTTPServer 9999</pre><h1>


Outer Frame. Hello!

<br/>

<iframe id="our-frame" style="width: 100%;"></iframe>

<script src="script.js"></script>

script.js

(function() {
  var iframe = document.querySelector('iframe#our-frame');

  // todo: use a setInterval fallback for older browsers
  if (!window.MutationObserver) {
    // older browsers don't get responsive iframe height, for now
    return;
  }

  console.log('attaching an iframe observer...\n\n');

  var iframeObserver = new MutationObserver(function(){
    // this code should run every time a mutation is detected
    // wherever we attach the observer:
    console.log('iframe mutation observed!');
  });

  var interval = setInterval(function(){
    if (iframe.contentWindow && iframe.contentWindow.document) {
      iframeObserver.observe(iframe.contentWindow.document, {
        attributes: true,
        subtree: true,
        childList: true,
        characterData: true
      });

      console.log('now that we have a handle on our inner frame and an observer is set up there, let\'s write some stuff to it. this should trigger a mutation, which should trigger an "iframe mutation observed!" log message.\n\nsince the observer should be attached, the same thing should also happen through the browser\'s console e.g. `document.querySelector(\'#our-frame\').contentWindow.document.write(\'foo\');`\n\n');

      iframe.contentWindow.document.write('hello');

      clearInterval(interval);
    }
  }, 100);
})();

did you find a solution for this? I'm unable to attach mutation observer to Iframe

I ended up delaying 100ms before attaching the observer in Chrome - the body DOM object changed at some point. Might work for you, if a bit ugly.