A TLD can have an A record, so in theory you could have a "0 character" domain. You could also use it for email, so a@co would be a valid email address if co. h

A TLD can have an A record, so in theory you could have a "0 character" domain. You could also use it for email, so a@co would be a valid email address if co. had an A or MX record associated with it.