There's two parts to this question, really.

1. Q: Which browsers support the "async" attribute on a script tag in markup? A: IE10p2+, Chrome 11+, Safari 5+, Firefox 3.6+

2. 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/

There's two parts to this question, really.

1. Q: Which browsers support the "async" attribute on a script tag in markup?

A: IE10p2+, Chrome 11+, Safari 5+, Firefox 3.6+

2. 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/