code: function isSuperSandboxedIframe() {
    const sacrificialIframe = window.document.createElement('iframe');
    try {
      sacrificialIframe.setAttribute('style', 'display:none');
      window.document.body.appendChild(sacrificialIframe);
      sacrificialIframe.contentWindow._testVar = true;
      window.document.body.removeChild(sacrificialIframe);
      return false;
    } catch (e) {
      window.document.body.removeChild(sacrificialIframe);
      return true;
    }
  }
docs: Return true if we cannot document.write to a child iframe (this implies no allow-same-origin)