[Willox]

What feature of DNS allows this? I'm probably wrong if that's the case.

Perhaps I'll check some other domains too.

[giovannibajo1]

https://tools.ietf.org/html/rfc7871

[nl]

That's an extension to let you use Google's DNS servers and still have CDNs work.

Without it the functionality described above will just work - it's only when you are using an unusual DNS setup that the extension is needed.