//CSS & HTML stuff window.layerElements = document.querySelectorAll('.layer') window.addEventListener('resize', windowResized) window.addEventListener('scroll', windowScrolled) windowResized() function windowResized() { window.windowHeight = window.innerHeight windowScrolled() } function windowScrolled() { for (var i = 0; i < layerElements.length; i++) { var positionFromTop = (layerElements[i].getBoundingClientRect().top + layerElements[i].getBoundingClientRect().bottom) / 2 if (positionFromTop - windowHeight <= 0) { layerElements[i].classList.add('revealedLayer') } } }