Complex sentence: The history of pop-punk can be traced back, of course, to the punk of the 1970s, which by the 1990s became quite mainstream with grunge bands and their "alternative" counterparts afterwards, such as Green Day.
Split into simpler sentences: The history of pop punk can be traced back to the punk of the 1970s. By the 1990s, it became quite mainstream with grunge bands and their "alternative" counterparts afterwards--such as Green Day.