Hmm. From a quick glance, your markup seems ok now; the XSL reference tag is correct, the XSL file exists at the indicated location and does not have any obvious syntax errors either… Classic case of “should work”
My hunch is a MIME type issue. Your feed is returned with the semantically correct mime header of
application/rss+xml whereas the most compatible header would be
application/xml (see discussions here, for example).
The feed you linked as an example in your first post uses the latter and there this markup works fine (both on Firefox* and Chromium; your feed is offered as a “download” by Firefox and simply displayed as source markup in Chromium – hence my suspicion this may be due to the MIME header).
*Firefox removed RSS support in December 2018, so I’m using the “Want my RSS” extension which applies its own style sheet and ignores XSL unless clicking on the “source” link.