FeedDemon and Atom
Nick Bradbury, FeedDemon developer, has posted his thoughts on the subject of Atom and valid XML: “FeedDemon will also support Atom, but if an Atom feed isn’t well-formed XML, FeedDemon will display an error rather than try to parse it.”
This reminds me of another point—NetNewsWire’s error-reporting user interface isn’t that good. I plan to improve it, to make errors more obvious. I’m also considering a preference where you could tell the RSS parser to require valid XML.