It's a popular presentation format for RSS feeds. And you don't need to somehow attach a Javascript polyfill to an XML feed to make it readable for humans
i do that frequently, especially on podcast feeds where i am not actually interested in the rss entries but in the linked audio files. the rss reader doesn't play those well.
Show me how a polyfill allows me to type https://example.com/rss.xml in the URL bar and have it transformed to HTML in the browser?