indexOfの単語マッチを上手に使う(sliceはnodelistにかけるとieで例外を出すの別途対応が必要のはず)

getElementsByClassName

document.getElementsByClassName = function (needle) {
    var allNodes = document.getElementsByTagName('*'),
        nodes    = Array.prototype.slice.call(allNodes),
        outArray = [],
        i,c,l;
    
    for (i = 0, l = nodes.length;  i < l; i++) {
        if (nodes[i].hasAttribute("class")) {
            c = " " + nodes[i].className + " ";
            if (c.indexOf(" " + needle + " ") !== -1) {
                outArray.push(nodes[i]);
            }
        }
    }
    return outArray;
};