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When it is viewed, it comes up in the browser as plain text.
However, on a different server, the same site and same RSS feed displays in Firefox in it's RSS viewer. So I don't think it is the feed causing the issue, but the server.
How do I set up the server to serve the RSS feed correctly to the browser?
Feeds aren't meant to be viewed in browsers, but some of them have incorporated stylesheets for viewing. Some browsers, like Opera, have feed readers built in. Another possibility is that the individual had their browser's MIME type for RSS set to open their feed reader/aggregator.