The definition of origin attribute in Web Messaging specification Editor's Draft from January 18, 2012 says at
http://dev.w3.org/html5/postmsg/#dom-messageevent-origin


> It represents, in server-sent events and cross-document 
> messaging, the origin of the document that sent the 
> message (typically the scheme, hostname, and port of 
> the document, but not its path or fragment identifier).

From this I suspect that the scheme could be anything: http, https, mailto, irc, etc. In the context a script I was wondering if 
 
    iframe.contentWindow.postMessage('message','http://dev.opera.com')

could be rewritten

    iframe.contentWindow.postMessage('message','//dev.opera.com')

allowing Web sites to work with or without https without having to rewrite the code. If this is authorized maybe, the paragraph could be modified with 

    It represents, in server-sent events and cross-document 
    messaging, the origin of the document that sent the 
    message (typically the scheme null string included, 
    hostname, and port of the document, but not its path or 
    fragment identifier). 

    Some examples of valid origin values

    http://foo.example.com
    https://foo.example.com
    //foo.example.com