web workers postMessage

I am trying to figure out Web Workers, and am having a terrible time
understanding what values are allowed to be sent and received by
postMessage. It doesn't seem like the spec spells that out anywhere. I've
checked both

http://www.whatwg.org/specs/web-apps/current-work/multipage/comms.html#dom-window-postmessage

and

http://dev.w3.org/html5/workers/#dedicated-workers-and-the-dedicatedworkerglobalscope-interface

I'm sure I could just be missing it, but it doesn't seem like it's specified
anywhere. It would be very useful to include this in the spec in both of
these relevant locations.

Thanks

Elisabeth Robson