http://lxr.mozilla.org/seamonkey/source/editor/libeditor/html/nsHTMLObjectResizer.cpp#273
http://lxr.mozilla.org/seamonkey/source/editor/libeditor/html/nsHTMLAnonymousUtils.cpp#317
nsIContent *rootContent = doc->GetRootContent();
newContent->SetNativeAnonymous(PR_TRUE);
newContent->SetParent(rootContent);
inside nsHTMLEditor::CreateAnonymousElement seems to work
http://lxr.mozilla.org/seamonkey/source/editor/libeditor/html/nsHTMLEditor.cpp#5950