A common user interaction idiom on desktops (popularized by the New York Times's Snowfall long-form piece) is to tie animations to the browser's scroll event. This is frequently fraught with peril on mobiles because scrolling triggers a memory-intensive redraw that often blocks JavaScript execution. While newer mobile operating systems handle this better than before (I'm looking mainly at you, iOS), it's often not safe to assume that a scroll-dependent animation will play properly and won't feel unperformant and janky on mobiles. Again, this has improved quite a lot with newer devices, but it's still never a bad idea to tie animation states to tap events (possibly delivered via a button) on mobiles.