The sample is missing the second parameter. Should be something like:

o.contentWindow.postMessage('Hello World', "*");

Without the second parameter IE8 RTM will fail with a javascript error: 'invalid argument'.

Don't use "typeof(window.postMessage)" to determine whether a browser supports the HTML5 postMessage feature - both Internet Explorer 6 and 7 return a value of "function" as the type!
It appears as though IE7 have some sort of undocumented function called postMessage that has a completely different purpose. It could cause conflicts if you're not careful when calling this function. In IE7 typeof(window.postMessage) returns 'object' Ref: http://gold-king.appspot.com/static/post-message-test.html
- Thangaraju Ramasamy

Don't use "typeof(window.postMessage)" to determine whether a browser supports the HTML5 postMessage feature - both Internet Explorer 6 and 7 return a value of "function" as the type!

It appears as though IE6 and IE7 have some sort of undocumented function called postMessage that has a completely different purpose. It could cause conflicts if you're not careful when calling this function.

[Is there a reference or test for this? All my tests have shown this to NOT be the case, IE6 and 7 return "undefined"]

In IE7 typeof(window.postMessage) returns 'object' Ref: http://gold-king.appspot.com/static/post-message-test.html

See http://blogs.msdn.com/ieinternals/archive/2009/09/16/Bugs-in-IE8-support-for-HTML5-postMessage-sessionStorage-and-localStorage.aspx for more details.

IE 8 Beta1 included this feature before the HTML5 proposal was changed. The IE8 Beta2 build will be updated to comply with the current HTML5 proposal.

This sample page demonstrates that IE8 Beta 1( I'm running 8.0.6001.17184 ) deviates significantly from the HTML5 spec for postMessage:

http://stevesouders.com/misc/test-postmessage.php

The specific issues:

The HTML 5 spec says, "If the value of the targetOrigin argument is neither a single U+002A ASTERISK character ("*") nor a valid URI or IRI, then throw a SYNTAX_ERR exception...". IE8 doesn't thrown an exception when the second parameter is missing.

The HTML 5 spec says, "If the value of the targetOrigin argument is neither a single U+002A ASTERISK character ("*") nor a valid URI or IRI, then throw a SYNTAX_ERR exception...". Even though "*" is valid according to the spec, IE8 still throws an exception and aborts.

The HTML 5 spec says, "When a script invokes the postMessage(message, targetOrigin) method on a Window object,...". But postMessage only works when it's attached to the document object in IE8.