I tried everything I could think of to style my RSS feed but nothing worked until I added a bunch of garbage at the top of the file to keep the browser from ignoring my instructions, and this was all the way back in 2020.
The two browsers you were complaining about are IE, long dead, and Firefox, which removed its feed reading functionality in 2018. I can only assume that you’re misremembering when you last tried this, because what you’re describing simply doesn’t happen any more.
You assume incorrectly. The earliest date in my rss file is August 2019, which is the test entry I made to make sure it worked.
Firefox dropped Live Bookmark support in December 2018, version 64.0. I was using Debian stable and Firefox ESR at the time, which was Firefox 60.9.
So I did get bitten by this, but I decided to try styling rss feeds at the exact wrong time.