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The DEFER attribute is an easy way to avoid the bad blocking behavior of scripts with the addition of a single word:

<script defer src='A.js'></script> 

Although DEFER is part of the HTML 4 specification, it is implemented only in Internet
Explorer and in some newer browsers.

stackoverflow上有一篇Defer Attribute (Chrome)中提到，chrome和firefox都已经支持了defer，但是opera还不支持。

在opera的网站找到这篇Timing and Synchronization in JavaScript提到：
There is a drawback to the synchronous loading of script blocks: If lots of code has to be downloaded and executed while parsing the head of the document, there might be a noticeable lag before the page begins to render.

To alleviate this, we could have used the defer attribute on script elements. This indicates that the browser is allowed to load the script asynchronously. However, we cannot be sure when the script actually will be executed, it could be before or after the page has finished rendering. Opera ignores the defer attribute completely.

不过文章已经是2007年的了。