After years of developing their style on the grassroots punk circuit of New England, The Mighty Mighty Bosstones perfected a ska-punk formula that eventually won mainstream acceptance and even produced a massive crossover single with 1997’s “The Impression That I Get.” Because the Bosstones were such an inherently lovable band even before they were discovered by the masses, they didn't need to reformat their sound in order to become victorious. Though there are certainly differences between early songs like 1994’s “Don’t Know How to Party” and late-career singles like 2000’s “So Sad to Say,” the band never deviated from their fundamental mission: to write perfect mergers of pop songs, fist-pumping punk anthems, and danceable and authentic ska. 20th Century Masters charts several of their best attempts, including “Kinder Words,” “The Rascal King,” and “Someday I Suppose,” but it also boasts some of the oddities that affirm this Boston band’s proud idiosyncrasy, among them the African-tinged “She Just Happened” and a cover of KISS’s “Detroit Rock City.”