markdown:

<script>
function shadow () {
  if (!document.body.attachShadow) return
  var shadows = document.querySelectorAll('div.shadow')
  for (var i = 0; i < shadows.length; i++) {
    var dom = shadows[i]
    if (!dom.attachShadow) continue
    var innerHTML = dom.innerHTML
    dom.innerHTML = ''
    var shadow = dom.attachShadow({ mode: 'open' })
    shadow.innerHTML = innerHTML
    dom.className = ''
  }
}
</script>

iframe: