Is this still the case in 2012? I imported bookmarks exported as Navigator and I see no tags

My feeds continue to fail Feed Validator especially when I have 2 tags selected

http://feedvalidator.org/check.cgi?url=http%3A%2F%2Fwww.diigo.com%2Frss%2Fuser%2FEmagin%2Feducation%2Benvironment

These feeds also won't load in certain feed renderers I use in Joomla.
Can someone explain what the status of feed validation is?
Thanks

I am trying to render rss feeds inside Joomla.

Most feeds work fine while all Popular or My rss feeds do not render properly.

For example:
This delic works fine
http://feeds.delicious.com/v2/rss/popular/nonprofit?count=15

This diigo does not
http://www.diigo.com/rss/tag/books?tab=151

When I run the diigo feeds through a validator, it always tells me that atom link is missing. Could this be the problem?
http://feedvalidator.org/check.cgi?url=http%3A%2F%2Fwww.diigo.com%2Frss%2Ftag%2Fbooks%3Ftab%3D151