It's a protection against scroll-behavior: smooth which messes things up. Some CSS libraries apply that, so we're overwriting it. In order for ScrollTrigger to do its magic, it must temporarily set the scroll position of the page back to the top (0), do all of its calculations and pre-optimize, and then restore the scroll position (it's invisible to the user), but if you apply scroll-behavior: smooth, that basically prevents that from working. It's sorta like setting a CSS transition to a value