AJAX-problem : Layout Isn't Applied On New Elements

I want to create a virtual filesystem. When you open page you see a
number of 'folders' represented by a name and an icon. When you click
on the icon an xmlhttprequest is send retrieving the subfolders and
files. When you click on thus created representation of a subfolder a
new xmlhttprequest is send to the server to retrieve the subfolders and
files for that subfolder.

On the serverside I create a domdocument (php) which contains html-tags
(div, h2, span, img, ...) and send it as xml to the client javascript.
I get the information (the html tags and attributes like src, id, href,
.... are ok) and take the root element and put in in the specified place
on my html-page. Only the titles of the subfolders (h2-tag) are shown
as text and they stick together instead of showing up in bold and
sperated like a good h2 should do. (I don't retrieve the files yet).

The layout isn't used, no images are shown, h2 is shown as plain text,
.... When I view the generated source with webdeveloper toolbar in
firefox everything looks fine, when I save generated source as html and
open it in browser the correct layout is applied.