在HTML5規范中定義，
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
也就是說 link的加載機制是由瀏覽器實現的，規范中並沒有指明是同步還是異步，
但是能夠保證的是 如果link沒有加載完成，document.onload不會被觸發
我們一般認為是 同步的