Which browsers support <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
- FF 3.6+
- FF for Android All Versions
- IE 10+ (starting with preview 2)
- Chrome 8+
- Chrome For Android All versions
- Safari 5.0+
- iOS Safari 5.0+
- Android Browser 3.0+ (honeycomb on up)
- Opera 15.0+
- Opera Mobile 16.0+
- Opera Mini None (as of 8.0)
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:
- FF 4+
- IE 10+ (preview 2 and up)
- Chrome 12+
- Chrome For Android 32+
- Safari 5.1+
- No android versions
There's two parts to this question, really.
Q: Which browsers support the "async" attribute on a script tag in markup?
A: IE10p2+, Chrome 11+, Safari 5+, Firefox 3.6+
Q: Which browsers support the new spec that defines behavior for the "async" property in JavaScript, on a dynamically created script element?
A: IE10p2+, Chrome 12+, Safari 5.1+, Firefox 4+
As for Opera, they are very close to releasing a version which will support both types of async. I've been working with them closely on this, and it should come out soon (I hope!).
More info on ordered-async (aka, "async=false") can be found here: http://wiki.whatwg.org/wiki/Dynamic_Script_Execution_Order
Also, to test if a browser supports the new dynamic async property behavior: http://test.getify.com/test-async/