If you want to style RSS, you can just go straight to JavaScript and avoid all the XSLT mishegas.
I made a site with to get people started: https://www.rss.style/
Example RSS feed: https://www.rss.style/changelog.xml
Cross-site is fine by default, though the script is small enough to easily self-host. If you have a content-security-policy, you'll need to allow the host in script-src.