Simple Multi Layer Parallax in Javascript

gistfile1.js

// <div class="parallax" data-speed="50" style="background-image:url(...)"></div>
// <div class="parallax" data-speed="75" style="background-image:url(...)"></div>

window.addEventListener("scroll", function(event){

	var top = this.pageYOffset;
	var layers = document.getElementsByClassName("parallax");
	
	var layer, speed, yPos;
	for (var i = 0; i < layers.length; i++) {
		layer = layers[i];
		speed = layer.getAttribute('data-speed');
		var yPos = -(top * speed / 100);
		layer.setAttribute('style', 'transform: translate3d(0px, ' + yPos + 'px, 0px)');
	}
});