Happy Eyeballs is primarily designed let users with broken networks continue connecting to dual-stack web servers. In some cases, it also covers up brokenness

Happy Eyeballs is primarily designed let users with broken networks continue connecting to dual-stack web servers. In some cases, it also covers up brokenness on the server end (e.g. publishing AAAA records to nowhere), but that's arguably more of a bug than a feature. Back in 2011, content providers were terrified of enabling IPv6, because the data showed that ~0.1% of users would drop off the map due to broken networks. Happy Eyeballs helped to break the logjam.