script.js

/**
 * Shuffle elements method
 * @param elems
 */
function shuffle(elems) {
    allElems = (function(){
        var ret = [], l = elems.length;
        while (l--) { ret[ret.length] = elems[l]; }
        return ret;
    })();

    var shuffled = (function(){
        var l = allElems.length, ret = [];
        while (l--) {
            var random = Math.floor(Math.random() * allElems.length),
                randEl = allElems[random].cloneNode(true);
            allElems.splice(random, 1);
            ret[ret.length] = randEl;
        }
        return ret;
    })(), l = elems.length;

    while (l--) {
        elems[l].parentNode.insertBefore(shuffled[l], elems[l].nextSibling);
        elems[l].parentNode.removeChild(elems[l]);
    }

}

document.addEventListener("DOMContentLoaded", function(event) {
    // on load shuffle everything
    shuffle( document.getElementsByClassName('pet') );
});