Ton gave me a suggestion to refer to the XSL file on my local domain. I did this, but still only saw the raw OPML appear. After some thinking, I thought about MIME types, and wondered if this was set up on my web hosting to serve OPML as XML. I uploaded a copy of the file with a .xml extension, and everything worked! I have found some resources for updating MIME types, will post later after some experimenting.