Message
The use of this DTD has been deprecated by Netscape
Explanation
Netscape is planning on removing this DTD as of July 1, 2007. This will cause a number of feed readers to fail to be able to process your feed.
Solution
One solution is 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.
An alternate solution is to take this opportunity to upgrade to a newer feed format, such as RSS 1.0, RSS 2.0, or Atom 1.0.
Not clear? Disagree?
Let us know on the feedvalidator-users discussion list!