It's HTTP headers not HTML as far as I could tell. The article didn't say the exact name of the header but it mentions support.torproject.org uses it so lookin

It's HTTP headers not HTML as far as I could tell. The article didn't say the exact name of the header but it mentions support.torproject.org uses it so looking into its headers: $ curl -I https://support.torproject.org/ [redacted] Onion-Location: http://4bflp2c4tnynnbes.onion/index.html [redacted]