Check the DOM for images that need to be loaded from the background page

content-script.js

function onCspCheckLoaded(isCspBlocking) {
  extension_module.csp = isCspBlocking; // cache the check value
  if (isCspBlocking) {
     // Capture error events triggered by img tags
    document.addEventListener('error', onExtensionImgError, true);
    // Check for background images
    var observer = new MutationObserver(withMutations);
    var whatToObserve = { childList: true, subtree: true };
    observer.observe(document, whatToObserve);
  }
}