The “authentic performance” movement was once viewed as a rebellious collection of obsessives, at once fascinating and annoying, but nothing that threatened to invade the larger world of classical performance. But these days, historically-informed performance is the norm for most ensembles, and while you don’t see a symphony orchestra tuning its instruments down to play Mozart, the days of hacking through 18th-century music as if it had been composed by Gustav Mahler are over. Meanwhile, the true period purists, such as the Toronto-based Tafelmusik, continue to soldier on, devoted to their idealistic view of music in context.