css-loader.js

var loading = {},
    loaded = {},
    head = document.getElementsByTagName('head')[0],
    runCallbacks = function (path, type) {
        var cbs = loading[path], cb, i = 0;
        delete loading[path];
        while (cb = cbs[i++]) {
            cb[type] && cb[type]();
        }
    },
    onSuccess = function (path) {
        loaded[path] = true;
        runCallbacks(path, 'success');
    },
    onError = function (path) {
        runCallbacks(path, 'error');
    };

/**
 * @exports
 * @param {String} path resource link
 * @param {Function} [success] to be called if the script succeeds
 * @param {Function} [error] to be called if the script fails
 */
module.exports = function (path, success, error) {
    if (loaded[path]) {
        success && success();
        return;
    }

    if (loading[path]) {
        loading[path].push({success: success, error: error});
        return;
    }

    loading[path] = [{success: success, error: error}];

    var link = document.createElement('link');

    link.setAttribute('rel', 'stylesheet');
    link.setAttribute('type', 'text/css');
    link.setAttribute('href', (location.protocol === 'file:' && !path.indexOf('//') ? 'http:' : '') + path);

    if ('onload' in link) {
        link.onload = function () {
            link.onload = link.onerror = null;
            onSuccess(path);
        };

        link.onerror = function () {
            link.onload = link.onerror = null;
            onError(path);
        };
    } else {
        link.onreadystatechange = function () {
            var readyState = this.readyState;
            if (readyState === 'loaded' || readyState === 'complete') {
                link.onreadystatechange = null;
                onSuccess(path);
            }
        };
    }

    head.insertBefore(link, head.lastChild);
}