If you accessed this web page using the URL http://josefßon.rocks your client supports IDNA2008. An IDNA2003 client would convert the name into http://josefsson.rocks instead, which is currently unresolvable.
This site is also available over https using a certificate signed by Let's Encrypt, so they appear to support IDNA2008 as well.
See räksmörgås.josefsson.org and libidn for more information about IDNA2003 and IDNA2008.