See https://www.kame.net/ for a basic browser test. If you see the dancing turtle, it means you're connected over IPv6.
It even works without Javascript.
Was sad to read this. Of course I understand his decision but it was a very useful site and helped IPv6 keep limping along its (barely) upward trajectory.
Hopefully someone will come along to carry the torch.
It sounds like they were providing more than testing ipv6. I'm not apposed to putting up a test site if it only has to work with browsers. I would imagine something as simple as 2 DNS records A and AAAA references to 2 images. Just 2 png/gif stickers that say "IPv4 works!" and "IPv6 works!" and placeholders that show what is expected. Would more be required?