The client shouldn't even attempt to get an AAAA record unless they have an IPv6 stack available. In that case, their client should try to look up an A record and get an NXDOMAIN error which the browser usually shows as "IP address could not be found". If the client does attempt IPv6, you'll get a stack error when trying to connect to the remote host because the kernel will reject the address family and you'll get something like "network is unreachable". Some clients will also degrade from AAAA to A on error so you'll get the NXDOMAIN error as above.
I wrote this a few years ago, but I feel like putting a dedicated IPv4 like 66.66.66.66 in the A record would inform the web browser or other software that the website is only reachable via IPv6, and a more informative error message could be displayed.
This would require a proper RFC of course, with support from IANA and web browsers.
