firefox and opera are the only browsers capable of loading scripts in parallel while preserving execution order. in all other browsers, scripts must be loaded sequentially.
//只有firefox和opera支持并行加载脚本，并保持执行顺序。其他游览器必须按顺序进行加载。
all browsers respect css specificity based on the order of the link elements in the dom, regardless of the order in which the stylesheets are actually downloaded.
//所有游览器根据link在样式表的顺序进行渲染，而非根据最终下载时间。
