The W3C Feed Validation Service is a very useful tool to validate the syntax of Atom or RSS feeds. I just used it to determine why my LinkedIn WordPress application could no longer connect to my WordPress RSS feed.
It turns out one of the WordPress plugins was causing the problem. Although the RSS feed validated, it gave me a few recommendations to remove code that may otherwise cause issues for some readers. I simply just disabled the plugin which automatically removed the problematic code, and now my LinkedIn WordPress application is again successfully able to read my WordPress RSS feed. The service uses the OpenSource FeedValidator software.