To understand why the atom feed of my website does not interact with browsers I tried to analyze other websites that have a feed.xml file obtained through the same plugin that I use: jekyll-feed.
First of all, thank you @TerminalAddict for participating in this topic.
I answer your questions in a nutshell: XSLT transforms the file to make it more readable to the human eye, is a style sheet and the origin of the .xml file remains evident to the machine; The HTTP status of my feed.xml is 200 (https://httpstatus.io).
I discovered why Firefox does not interact natively with feeds: Firefox’s built-in support for web feeds and Live Bookmarks will be removed starting with the release of Firefox version 64 in December 2018. Support Mozzilla
Opera’s dilemma remains open: let me bring two more examples: https://jekyllrb.com/feed.xml (use jekyll-feed, if you try to open the file with Opera you will have a black screen); https://18f.gsa.gov/feed.xml (use jekyll-feed and Opera correctly displays the XML file, not a black screen).
Is it common to expect a browser to do something with a feed meant to be machine readable? What’s the use case of wanting to browse to a .xml or .json file with your browser versus a feed reader?
Right observation @mmistakes. The fact is that many Jekyll websites, including mine, present an icon in plain sight with a direct link to the feed.xml. All browsers allow you to view the xml files of sitemaps that are for the exclusive use of bots. Instead, the Opera browser does not allow you to view the vast majority of feed.xml and when a visitor presses on the link he is in front of a dark screen. We do not have to assume that everyone knows what is an RSS feed, I live in Italy, for example, and the target of my visitors is over 50: the computer literacy in my country occurred much later than other countries such as for example The United States of America.
I think the best solution in my case is to write an RSS page in html (to explain what the RSS feeds are and what to do to subscribe them) and keep my feed.xml hidden.