Contentwindow and Contentdocument

"Contentdocument"

W3C standard tells us that the document object can be returned through the Contentdocument property of the DOM object.
doc= document.getElementById (' J_mainframe '). Contentdocument
Ie6,ie7 are not supported, IE8 start support, need so access document.frames[' J _mainframe '].document;

"Contentwindow"
The Contentwindow property, which is a read-only property, returns the Window object for the specified IFRAME.
Although it is not part of the standard, it is supported by various mainstream browsers.

Contentwindow and Contentdocument