How to access a from the HTML embedded by

How to access a from the HTML embedded by

I have a webpage that has several <div> sections ("navcol" and "maincol").
The "navcol" div contains <object data="mypage.html">. Inside mypage.html
there are buttons the user can click to select pages to be inserted into
"maincol". I need to gain access to the innerHTML of "maincol" in the main
page with Javascript, so I can do the insertion (like using an iframe).
Can someone put me on the right track?
PS. I am using <object>, because I want to have HTML 4.0 "Strict" and
Iframes are not in "Strict", but s are.