Passing Info From One Page To Another

I've been Googling & can't find a consistent cross-browser (IE & Firefox) answer to this...

I need to pass some data from inside an iframe, to the parent page

I've got the parent page PAGE1.HTM which contains:

<div ID="mydata"></div>
:
<iframe src="page2.htm"></iframe>

And, inside PAGE2.HTM I will set mydata to a hard string. But in page2.htm do I do

var parent.document.mydata.innerHTML = "my data here'

or

var window.parent.document.all.mydata.innerHTML = "my data here"

or something else...??? to be cross-browser compatible (IE and Firefox mainly).