title: Can a website prevent an element from being added to the already loaded page?
body: <p>If the code below is run in DevTools or in the browser's address bar, the <code>h1</code> element will be inserted at the end of a website's page.</p>
  <pre><code>const elm = document.createElement('h1')
  const txt = document.createTextNode('New text')
  elm.appendChild(txt)
  document.body.appendChild(elm)
  </code></pre>
  <p>Can any site prevent this from happening using, for example, <code>MutationObserver</code>?</p>