Does it work with IPv6?
Not by default as it seems the content server is IPv4 enabled only. I suppose the Ookla-based tool would work over IPv6 also if the content server was setup for IPv6.
Speedtest.net's tool works over IPv6 if the content server is IPv6 enabled.
Broadbandreports.com's tool reportedly works over IPv6 if you enable Java to use IPv6 and the content server is IPv6 enabled.