The source code

    /**
     * @ignore
     * script/css load across browser
     * @author yiminghe@gmail.com
     */
    (function (S) {

        var CSS_POLL_INTERVAL = 30,
            UA= S.UA,
            utils = S.Loader.Utils,
        // central poll for link node
            timer = 0,
            monitors = {
                // node.id:{callback:callback,node:node}
            };

        /**
         * @ignore
         * References:
         *  - http://unixpapa.com/js/dyna.html
         *  - http://www.blaze.io/technical/ies-premature-execution-problem/
         *
         * `onload` event is supported in WebKit since 535.23
         *  - https://bugs.webkit.org/show_activity.cgi?id=38995
         * `onload/onerror` event is supported since Firefox 9.0
         *  - https://bugzilla.mozilla.org/show_bug.cgi?id=185236
         *  - https://developer.mozilla.org/en/HTML/Element/link#Stylesheet_load_events
         *
         * monitor css onload across browsers.issue about 404 failure.
         *
         *  - firefox not ok（4 is wrong）：
         *    - http://yearofmoo.com/2011/03/cross-browser-stylesheet-preloading/
         *  - all is ok
         *    - http://lifesinger.org/lab/2011/load-js-css/css-preload.html
         *  - others
         *    - http://www.zachleat.com/web/load-css-dynamically/
         */

        function startCssTimer() {
            if (!timer) {
                // S.log('start css polling');
                cssPoll();
            }
        }

        // single thread is ok
        function cssPoll() {

            for (var url in monitors) {

                var callbackObj = monitors[url],
                    node = callbackObj.node,
                    exName,
                    loaded = 0;
                if (UA.webkit) {
                    // http://www.w3.org/TR/DOM-Level-2-Style/stylesheets.html
                    if (node['sheet']) {
                        S.log('webkit loaded : ' + url);
                        loaded = 1;
                    }
                } else if (node['sheet']) {
                    try {
                        var cssRules = node['sheet'].cssRules;
                        if (cssRules) {
                            S.log('same domain firefox loaded : ' + url);
                            loaded = 1;
                        }
                    } catch (ex) {
                        exName = ex.name;
                        S.log('firefox getStyle : ' + exName + ' ' + ex.code + ' ' + url);
                        // http://www.w3.org/TR/dom/#dom-domexception-code
                        if (// exName == 'SecurityError' ||
                        // for old firefox
                            exName == 'NS_ERROR_DOM_SECURITY_ERR') {
                            S.log(exName + ' firefox loaded : ' + url);
                            loaded = 1;
                        }
                    }
                }

                if (loaded) {
                    if (callbackObj.callback) {
                        callbackObj.callback.call(node);
                    }
                    delete monitors[url];
                }

            }

            if (S.isEmptyObject(monitors)) {
                timer = 0;
                // S.log('end css polling');
            } else {
                timer = setTimeout(cssPoll, CSS_POLL_INTERVAL);
            }
        }

        S.mix(utils, {
            pollCss: // refer : http://lifesinger.org/lab/2011/load-js-css/css-preload.html
            // 暂时不考虑如何判断失败，如 404 等
                function (node, callback) {
                    var href = node.href,
                        arr;
                    arr = monitors[href] = {};
                    arr.node = node;
                    arr.callback = callback;
                    startCssTimer();
                }

        });
    })(KISSY);