Don't come up with any other variants. The above form is what is in general use around the internet and what some people will at least try to use in cases where a DNS label has both an AAAA and A and one of them doesn't work.
Where did the www.ipv6 and www.ipv4 "standard" come from?
He already explained that as quoted above. It is a de-facto standard since that is what is in general use around the Internet. Standards are not always created by standards groups, sometimes they just grows, like Topsy. The key thing here is that adding AAAA records for a host that also has an A record, can cause strange things to happen. If this would be bad for the services offered by the host with the A record, then you can create two new pseudo-hosts ipv6.host and ipv4.host. Put the AAAA record only on the ipv6.host entry, and make the ipv4.host entry either a duplicate of, or a CNAME to the original host with the A record. That way, you can still get some IPv6 traffic from IPv6 knowledgeable people for testing purposes. Or you can solicit people to help with your testing by using the ipv6.host variant. --Michael Dillon