editing another DOM document

Hello!
I've encountered the following problem:
I would like to open an existing html document using
javascript:
newDoc = window.open("myExistingDocument.html");

and to access and edit its own elements.
Like, when I am able to do
document.getElementById('myId').appendChild - 
which means editing the current document which holds
the script - I also want to do :
newDoc.document getElementbyId...,
that is to edit another html document different of the
one which contains the script, and to append info
THERE.
How is it possible to do?
Sincerely, Ira.


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