HTML 5 window.showModalDialog parameters

The sFeatures<http://msdn.microsoft.com/en-us/library/ms536759(VS.85).aspx> (IE) or options<https://developer.mozilla.org/en/DOM/window.showModalDialog> (FF) parameter to window.showModalDialog appears to be missing in the HTML 5 spec for this API.  I'm wondering how the user of the API will then be able to control the size and placement of the dialog, for example?  I'm also curious as to why the signature does not match the currently implemented version of the API in most major browsers (Chrome, Safari, IE, FF)?

Thanks

Jeremy Smith