动态创建的link标签不会阻塞其后动态创建的script的加载与执行

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

link.href = "http://udacity-crp.herokuapp.com/style.css?rtt=2"; 

link.rel = "stylesheet"; document.head.appendChild(link); 

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

script.src = "http://udacity-crp.herokuapp.com/time.js?rtt=1&a"; 

document.head.appendChild(script);