"How can such changes occur to the DOM that aren't brought about by other code on the page?" - The other code that made the change may have been written by ano

"How can such changes occur to the DOM that aren't brought about by other code on the page?" - The other code that made the change may have been written by another team or a third-party organization - The code setting the MutationObserver may not originate from the page e.g. a browser extension or bookmarklet