<iframe></iframe>
body{
 background:gray;
}

iframe{
  background:#fff;
}
const blob = new Blob(['<!DOCTYPE html><html><head><meta charset="utf-8"><title></title></head><body>hello world!!!</body></html>'],{ "type" : "text/html" })

const iframeDom = document.getElementsByTagName('iframe')[0]
const iframeSrc = URL.createObjectURL(blob)

iframeDom.src = iframeSrc
Rerun