I think it would be great for jschon to implement the built-in formats, and ultimately to support the format-assertion vocabulary, which requires full validation support for all the formats described in the spec. Up until now I've chosen to leave format validation up to the user, rather than implementing an arbitrary selection of formats which may be useful to some and not to others. Format validators are time consuming to do, and potentially tricky as you've pointed out with IP addresses, and they are not a top priority in my own use cases for the library.

But my feeling is that if you and/or others are keen to contribute individual format validators then let's go ahead and start ticking them off. We'll get them all done eventually!

Regarding the format validation example, thanks for pointing out the missed case. I'm open to suggestions as to how to fix the example - although I don't have time to look into the details of those formats any time soon. I'll be happy to accept PRs for new or modified examples.