Simple parallax background scrolling with jQuery

parallax-scroll.js

$(function() {
    var $el = $('.parallax-background');
    $(window).on('scroll', function () {
        var scroll = $(document).scrollTop();
        $el.css({
            'background-position':'50% '+(-.4*scroll)+'px'
        });
    });
});

parallax-scroll.scss

.parallax-background {
    background-image: url('http://placekitten.com/1000/1000');
    background-position: 50% 0;
    background-repeat: no-repeat;
    -webkit-background-size: cover;
    background-size: cover;

    -moz-transform: translate3d(0,0,0);
    -webkit-transform: translate3d(0,0,0);
    transform: translate3d(0,0,0);
}