Use JScript to calculate width iFrame should be (Safari give problem)

[StrangerInBeijing]

Hi,
I use an iFrame on my page, to load other pages in it.
I set the width of the iFrame to 100% and all works well.
But just now I realized that if you do not use a fixed with, the iFrame does not size in Safari (the width)

So I thought maybe I should use a javascript like this?

document.getElementById("body").width = document.getElementById("bodycell").width;

where "body" is the id of the iFrame, and "bodycell" is the <td> I placed the iFrame in.
But this code still set in percentance. If I say:

alert(document.getElementById("bodycell").width);

then i get "100%", not a fixed width

Any workaround?