<script>
window.onload = () => {
  b.addEventListener('DOMSubtreeModified', () => {
    var o = document.getElementById('a')
    var a = o.attributes
    for (let j = 0; j < a.length; j++) {
      o.setAttribute(a[j].name, 'i')
      o.parentNode.appendChild(o)
    }
  })
  b.setAttribute('a', b)
}
</script>
<iframe id='a' sandbox='' allowfullscreen=''></iframe>
<dfn id='b'>