Hi Jonathan,
HTMLUI does not include support for loading and displaying XML files. It is possible to use a generic XSLT transformer from XML to HTML and CSS and then load the HTML file.
I have created a sample that uses a specific XSLT file and transforms the XML file to HTML before loading it into HTMLUI.
http://www.syncfusion.com/support/user/uploads/htmluixsltransform.zip
We will also look into support displaying XML as HTML without a XSLT file.
Thanks,
Davis