Just released a huge update to Mess With DNS (https://messwithdns.net) -- it looks the same, but now it's powered by a real DNS implementation on the backend (PowerDNS) and should be MUCH better at being standards compliant.
When we released Mess With DNS 3 years ago, the DNS implementation was pretty crappy and over the years I've gotten a lot of bug reports about it, so I’m very excited to replace my crappy implementation.
Would love to hear about any problems you run into!