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.

  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/


A comprehensive list of browser versions supporting the async parameter is available here

Tags:

Javascript

Html

Performance

Asynchronous

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