Question: This would seem to be the case in Firefox 3.5+, there I can instantiate a Web Worker, and inside the worker, spawn another thread. However, the code will not work in Google Chrome, leading me to believe this is either a Mozilla proprietary extension to the spec or that Chrome has not fully implemented the spec. So which is it?

Answer: 
<http://www.whatwg.org/specs/web-workers/current-work/#interface-objects-and-constructors> seems to say it **MAY** be possible (as of 21.10.2009):

"There must be no interface objects and constructors available in the global scope of scripts whose script's global object is a WorkerGlobalScope object except for the following: [...] The Worker and SharedWorker interface objects and constructors."

Here

* "in the global scope of scripts whose script's global object is a WorkerGlobalScope object" means "among the worker's globals"
* "must be no ... except" is open to the interpretation.

You might want to search open issues/post feedback. The spec has this text:

```
To send feedback: whatwg@whatwg.org
To view and vote on feedback: http://www.whatwg.org/issues/

```
Yes, webworkers can spawn subworkers. major latest browsers are supporting webworker interface.

Here are some tutorials to get it.

<https://developer.mozilla.org/en-US/docs/DOM/Using_web_workers>

<https://developer.mozilla.org/en-US/docs/DOM/Worker/Functions_available_to_workers>