Currently, the checks specified for MessagePort.postMessage() are different from the checks done in window.postMessage() (as described in section 7.2.4 "Posting messages with message ports").

In particular, step 4 of section 7.2.4 says:

If any of the entries in ports are null, if any of the entries in ports are not entangled MessagePort objects, or if any MessagePort object is listed in ports more than once, then throw an INVALID_STATE_ERR exception.

The spec for MessagePort.postMessage() does not throw an exception if any of the entries in ports are not entangled (per this thread). We should probably update the spec for window.postMessage() to define the same behavior there as well.

Also, as written, the spec now incorrectly lets us send a cloned port multiple times. So code like this would not generate an error:

var channel = new MessageChannel();
otherWindow.postMessage("message1", channel.port1);
otherWindow.postMessage("message2", channel.port1);   // Sent the same port again

The current WebKit behavior is to throw an INVALID_STATE_ERR in this case, while still allowing closed ports to be sent, which I believe is the intended behavior based on previous discussions. If this is correct, we should update the spec to prohibit resending cloned ports.