It's not zero effort at all. For XML(which RSS is) with xlst it is serving only 2 static files. The XML file with a reference to the xlst file, and the xlst file.

The XML can be read without transformation by tools like RSS readers, but displayed with transformation into HTML for viewing in a browser with the xlst.

You're saying it is easy to polyfill, but involving JavaScript at all completely breaks the (useful) paradigm