parallax.js

window.onscroll = function() {
  var speed = 4.0;
  var ypos = -window.pageYOffset / speed + "px";
  var xpos = "50%";
  var elems = document.getElementsByClassName('parallax');
  for (var i = elems.length - 1; i >= 0; i--) {
    elems[i].style.backgroundPosition = xpos +" "+ ypos;
  };
}