Problem Adding HTML Content Loaded Using Document.load()

I'm building a tree control that lazily loads branches of the tree
using the document.load() method.

The external XML document that is loaded is generated by a servlet as
XHTML. What I would like to do is to add the new tree branch to the
correct place in the document in the browser window using the innerHTML
property of the parent node.

The relevant code snippet that I've written to do this is:

var target = document.getElementById(parentId);
target.innerHTML = xmlDoc.documentElement.xml;

This doesn't work, furthermore looking at the W3C DOM API it would seem
that the document.xml property is a Microsort proprietary extension
which makes it unsuitable for my application.