JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdn.jsdelivr.net/npm/diff-dom@5.0.7/browser/diffDOM.js"></script>
<html>
  <body>
    <iframe id="iframe" srcdoc="<html><body><p>iframe content</p></body></html>"></iframe>
    <p id="output"></p>
  </body>
</html>

JavaScript

document.addEventListener("DOMContentLoaded", function() {
  let iframe = document.getElementById('iframe');
  let output = document.getElementById('output');
  iframe.onload = function() {
    let innerDoc = iframe.contentWindow.document;
    let textNode = innerDoc.createTextNode('foo');
    document.adoptNode(textNode);
    output.append('> textNode instanceof textNode.ownerDocument.defaultView.Text');
    output.append(document.createElement('br'));
    output.append('< ' + (textNode instanceof textNode.ownerDocument.defaultView.Text));
    output.append(document.createElement('br'));
    output.append('> textNode instanceof innerDoc.defaultView.Text');
    output.append(document.createElement('br'));
    output.append('< ' + (textNode instanceof innerDoc.defaultView.Text));
  }
});