Greasemonkey script to look for duplicate HTML id attributes

idValidator.user.js

// ==UserScript==
// @name HTML ID attribute validator
// @description Looks for duplicate IDs in DOM and alerts them
// @namespace all
// @grant none
// @include /^https?://.*/
// @license GPLv3 or any later version (http://www.gnu.org/copyleft/gpl.html)
// @downloadURL https://gist.github.com/antirais/8853684/raw
// @updateURL https://gist.github.com/antirais/8853684/raw
// @version 1.3.3
// @author Anti Räis
// ==/UserScript==

// INSTALL: to install script while viewing the gist in GitHub, click "Raw".
// It should open Greasemonkey confirmation box, then click "Install"

var findDuplicates = function() {
    var ids = {};
    var duplicates = {};
    var nodes = document.querySelectorAll('[id]');
    var nodes_count = nodes.length;
    var start = Date.now();

    if (!nodes_count) return;

    while (nodes_count--) {
        var node_id = nodes[nodes_count].getAttribute('id');
        if (node_id in ids) {
            if (node_id in duplicates) {
                duplicates[node_id] += 1;
            } else {
                duplicates[node_id] = 2;
            }
        } else {
            ids[node_id] = node_id;
        }
    }

    console.log("%d attributes scanned in %d ms", nodes.length, (Date.now() - start));

    if (Object.getOwnPropertyNames(duplicates).length) {
        alert("Duplicates: " + JSON.stringify(duplicates, null, 4));
    }
}

document.addEventListener('DOMContentLoaded', findDuplicates, false);