在HTML5规范中定义， section of the HTML5 spec
User agents may opt to only try to obtain such resources when they are needed, instead of pro-actively fetching all the external resources that are not applied
The [<link>] element must delay the load event of the element's document until all the attempts to obtain the resource and its critical subresources are complete
也就是说 link的加载机制是由浏览器实现的，规范中并没有指明是同步还是异步，
但是能够保证的是 如果link没有加载完成，document.onload不会被触发
我们一般认为是 同步的