chrome/browser/resources/new_tab_page/iframe.html

<style>
  :host(:not([hidden])) {
    display: block;
  }

  #iframe {
    border: none;
    border-radius: inherit;
    display: block;
    height: inherit;
    max-height: inherit;
    max-width: inherit;
    width: inherit;
  }
</style>
<iframe id="iframe" src="[[src_]]" allow="[[allow]]"></iframe>