Site is great, service is great. :)
  One minor possible enhancement: why not following the guidelines for a free JavaScript, so that the Mozilla LibreJS plugin by GNU does not block your validator/script.js JavaScript?

The background and rationale of this is https://www.gnu.org/licenses/javascript-labels-rationale.html

A brief description of "JavaScript Web Labels" is https://www.gnu.org/licenses/javascript-labels.html

The full guidelines to free your JavaScript are https://www.gnu.org/software/librejs/free-your-javascript.html

Simple, aren't they? You only have to modify your script.js header from the current one to

/*
 *
 * @source: http://validator.nu/script.js
 *
 * @licstart  The following is the entire license notice for the 
 *  JavaScript code in this page.
 *
 * Copyright (c) 2005-2007 Henri Sivonen
 * Copyright (c) 2007-2010 Mozilla Foundation
 * Copyright (c) 2007 Simon Pieters
 *
 * Permission is hereby granted, free of charge, to any person obtaining a
 * copy of this software and associated documentation files (the "Software"),
 * to deal in the Software without restriction, including without limitation
 * the rights to use, copy, modify, merge, publish, distribute, sublicense,
 * and/or sell copies of the Software, and to permit persons to whom the
 * Software is furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
 * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 * DEALINGS IN THE SOFTWARE.
 *
 * @licend  The above is the entire license notice
 * for the JavaScript code in this page.
 *
 */

This should work. What do you think?