The use of this DTD has been deprecated by Netscape
Netscape is stopped forwarding requests for this DTD as of July 1, 2007. This will cause a number of feed readers to fail to be able to process your feed.
The simplest solution is to update the link to http://www.rssboard.org/rss-0.91.dtd.
Another solution may be to remove the reference to the DTD entirely. If you elect to do this simply be sure that none of the named entity references defined in the DTD are used in your feed. Once you remove the DOCTYPE, you can use the Feed Validator to check for such references.
Not clear? Disagree?
You might be able to find help in one of these fine resources.