Provides getElementsByClassName method... if native implementation is not detected.

/**
 * Provides getElementsByClassName method, if there is no native
 * implementation of this method.
 *
 * NOTE that there are limits and differences compared to native
 * getElementsByClassName as defined by e.g.:
 *   https://developer.mozilla.org/en/DOM/document.getElementsByClassName
 *   http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#dom-getelementsbyclassname
 *   http://www.whatwg.org/specs/web-apps/current-work/multipage/dom.html#dom-document-getelementsbyclassname
 *
 * Namely, this implementation supports only single class name as
 * argument and not set of space-separated tokens representing classes,
 * it returns Array of nodes rather than live NodeList, and has
 * additional optional argument where you can limit search to given tags
 * (via getElementsByTagName).
 *
 * Based on
 *   http://code.google.com/p/getelementsbyclassname/
 *   http://www.dustindiaz.com/getelementsbyclass/
 *   http://stackoverflow.com/questions/1818865/do-we-have-getelementsbyclassname-in-javascript
 *
 * See also http://ejohn.org/blog/getelementsbyclassname-speed-comparison/
 *
 * @param {String} class: name of _single_ class to find
 * @param {String} [taghint] limit search to given tags
 * @returns {Node[]} array of matching elements
 */
if (!('getElementsByClassName' in document)) {
    document.getElementsByClassName = function (classname, taghint) {
        taghint = taghint || "*";
        var elements = (taghint === "*" && document.all) ?
                       document.all :
                       document.getElementsByTagName(taghint);
        var pattern = new RegExp("(^|\\s)" + classname + "(\\s|$)");
        var matches= [];
        for (var i = 0, j = 0, n = elements.length; i < n; i++) {
            var el= elements[i];
            if (el.className && pattern.test(el.className)) {
                matches[j] = el;
                j++;
            }
        }
        return matches;
    };
} // end if