On December 1, 2009, Google announced support for asynchronous Google Analytics tracking.
The asynchronous tracking is achieved using the async directive for the
<script async="async" />
The async support as specified by google is achieved using two parts:
using script on your page (the script is supplied by google) to write out a <script> tag to the DOM.
that script has async="true" attribute to signal to compatible browsers that it can continue rendering the page.
The first part works on browsers without support for
<script async.. tags, allowing them to load async with a "hack" (although a pretty solid one), and also allows rendering the page without waiting for ga.js to be retrieved.
The second part only affects compatible browsers that understand the async html attribute
The "html5 proper" way to specify async is with a
<script async src="...", not
<script async="true". However, initially browsers did not support this syntax, nor did they support setting the script property on referenced elements. If you want this, the list changes: