We measure YouTube content delivery over IPv6 using ?100 Sam-Knows probes connected to dual-stacked networks representing 66 different origin ASes. Using a 34-months long (Aug 2014-Jun 2017) dataset, we show that success rates of streaming a stall-free version of a video over IPv6 have improved over time. We show that a Happy Eyeballs (HE) race during initial TCP connection establishment leads to a strong (more than 97%) preference over IPv6. However, even though clients prefer streaming videos over IPv6, we observe worse performance over IPv6 than over IPv4. We witness consistently higher TCP connection establishment times and startup delays (?100 ms or more) over IPv6. We also observe consistently lower achieved throughput both for audio and video over IPv6. We observe less than 1% stall rates over both address families. Due to lower stall rates, bitrates that can be reliably streamed over both address families are comparable. However, in situations, where a stall does occur, 80% of the samples experience higher stall durations that are at least 1s longer over IPv6 and have not reduced over time. The worse performance over IPv6 is due to the disparity in the availability of Google Global Caches (GGC) over IPv6. The measurements performed in this work using the youtube test and the entire dataset is made available  to the measurement communityGrant:-We like to thank Sam Crawford and Jamie Mason for providing us support on the SamKnows infrastructure and to all volunteers who host a probe for us. This work was supported by the European Commission Horizon 2020 Programme RIFE Project Grant No. 644663. This work was also supported by the European Communitys Seventh Framework Programme in (FP7/2007-2013) Grant No. 317647 (Leone) and funded by Flamingo, a Network of Excellence project (ICT-318488) supported by the European Commission under its Seventh Framework Programme.