##
serverFrame.contentWindow

    w = s.contentWindow

The "contentWindow" property points to the actual HTML page that is sent to the server by a source serverFrame, or sent back from the server in a target serverFrame. All elements in that page are derived from "contentWindow", e.g. the first form element is:

    s.contentWindow.document.body.getElementsByTagName("form")[0]

Normally, a serverFrame will only contain one form with hidden variables that can be set and read by "setVar()" and "getVar()". On "submit()" the form will be sent to the server, with the hidden variables as CGI parameters. On return to a serverFrame that is set as target for a server operation, the contentWindow can contain anything the server has sent back. Normally this should be another form with hidden variables that can be read by "getVar()". Single HTML elements like tables or images are also good ways to send information back. Once arrived in the browser, they can by copied to visible locations on the screen in the serverFrame's "onReturn()" method.
If a returned serverFrame's contentWindow does not contain a form, an empty form is automatically created in it, so "getVar()" and "setVar()" can be used on it.