You don't need to have the ></Mapping> for each element. It can be shortened to /> as such:
<Mapping Art AssetID="some_doodad" FileName="some_doodad.tga" />
It works for self-contained elements in PanelFiles as well:
<Element ID="Element" X="0" Y="0" Width="0" Height="0">
<Element ID="SubElement" X="0" Y="0" Width="0" Height="0" />
It really serves no purpose other than to make the code look a little cleaner and save a few kb if it's a big file, but I thought I'd mention it.
I guess I never noticed the thing about the space before the Mapping calls because I indent everything with at least one tab (equiv of 4 spaces for me... I use notepad++). I don't know why that would make a difference, but it's definitely worth a look if the offending code all starts at the beginning of a line. Another thing I just noticed is that you *need* a space between the ArtAssetID and the FileName or the whole skin will fail to load. ArtAssetID="Something"FileName="Something.tga" will cause LoTRO to ignore the skin completely. ArtAssetID="Something" FileName="Something.tga" works. I had one line in my compilation test UI that didn't have a space between the two and the UI failed to load until I added it. (sorry for the derails Gaylen