The source code

    /**
     * @ignore
     *  solve io between sub domains using proxy page
     * @author yiminghe@gmail.com
     */
    KISSY.add('ajax/sub-domain-transport', function (S, XhrTransportBase, Event, DOM) {

        var PROXY_PAGE = '/sub_domain_proxy.html',
            doc = S.Env.host.document,
            iframeMap = {
                // hostname:{iframe: , ready:}
            };

        function SubDomainTransport(io) {
            var self = this,
                c = io.config;
            self.io = io;
            c.crossDomain = false;
            S.log('use SubDomainTransport for: ' + c.url);
        }

        S.augment(SubDomainTransport, XhrTransportBase.proto, {
            // get nativeXhr from iframe document
            // not from current document directly like XhrTransport
            send: function () {
                var self = this,
                    c = self.io.config,
                    uri = c.uri,
                    hostname = uri.getHostname(),
                    iframe,
                    iframeUri,
                    iframeDesc = iframeMap[hostname];

                var proxy = PROXY_PAGE;

                if (c['xdr'] && c['xdr']['subDomain'] && c['xdr']['subDomain'].proxy) {
                    proxy = c['xdr']['subDomain'].proxy;
                }

                if (iframeDesc && iframeDesc.ready) {
                    self.nativeXhr = XhrTransportBase.nativeXhr(0, iframeDesc.iframe.contentWindow);
                    if (self.nativeXhr) {
                        self.sendInternal();
                    } else {
                        S.error('document.domain not set correctly!');
                    }
                    return;
                }

                if (!iframeDesc) {
                    iframeDesc = iframeMap[hostname] = {};
                    iframe = iframeDesc.iframe = doc.createElement('iframe');
                    DOM.css(iframe, {
                        position: 'absolute',
                        left: '-9999px',
                        top: '-9999px'
                    });
                    DOM.prepend(iframe, doc.body || doc.documentElement);
                    iframeUri = new S.Uri();
                    iframeUri.setScheme(uri.getScheme());
                    iframeUri.setPort(uri.getPort());
                    iframeUri.setHostname(hostname);
                    iframeUri.setPath(proxy);
                    iframe.src = iframeUri.toString();
                } else {
                    iframe = iframeDesc.iframe;
                }

                Event.on(iframe, 'load', _onLoad, self);

            }
        });

        function _onLoad() {
            var self = this,
                c = self.io.config,
                uri = c.uri,
                hostname = uri.getHostname(),
                iframeDesc = iframeMap[hostname];
            iframeDesc.ready = 1;
            Event.detach(iframeDesc.iframe, 'load', _onLoad, self);
            self.send();
        }

        return SubDomainTransport;

    }, {
        requires: ['./xhr-transport-base', 'event', 'dom']
    });