I think realistically the only way to resolve this is to formally support and define a specification for feed formats. The available mime types lists two formats that I think are important here. text/plain and text/markdown. I believe a specification that defines and formalizes this so that a feed author can state in their feed that their feed is primarily text/plain or text/markdown or via HTTP headers (not mandatory) will work here. I also think it might be worthwhile niversing this and defaulting to text/plain (by design and by default, spec TBD) and then clients like yanrd can just be updated to declare text/markdown.