A*Teens debuted in 1999 as pop’s biggest ABBA fans—their debut album, The Abba Generation, showcased the quartet’s harmonies on covers of their fellow Swedes’ smashes like “Mamma Mia” and “Super Trouper”. After topping charts around the world, Dhani Lennevald, Sara Lumholdt, Amit Paul and Marie Serneholt began forging their own pop path. “Upside Down”, the lead single from their 2001 album Teen Spirit, took all the right lessons from ABBA: sing-along-ready lyrics and a bright chorus. Subsequent singles like the swaggering “Floorfiller” and the spunky “A Perfect Match” added turn-of-the-century teen-pop flair to the mix.