I have validated some RSS files using web-service.
http://feedvalidator.org/
and
http://validator.w3.org/feed/
and now, the feedvalidator.org's feedvalidator is on this site too.
http://www.garterblue.com/feedvalidator/
You SHOULD check "Maintain Design Notes" in the Site Definition dialog box to use this extension.
Because this extension save some item using Designnotes.
If you don't check it, you have to input some items every time.
Some definitions are saved to Designnotes of /index.*.
At first, search the file named index.rsssan on site root. When there isn't this file, index.html, index.htm...
<?xml version="1.0" encoding="utf-8" ?>
<info>
<infoitem key="RSS3_SITE_TITLE" value="garterblue.com" />
<infoitem key="RSS3_SITE_URL" value="http://www.garterblue.com/" />
<infoitem key="RSS3_SITE_DESCRIPTION" value="Honi soit qui mal y pense." />
<infoitem key="RSS3_SITE_MANAGER" value="gb" />
<infoitem key="RSS3_SITE_URL_RSS10" value="http://www.garterblue.com/com.garterblue.rss3.xml" />
<infoitem key="RSS3_SITE_UTC_FORMAT" value="5" />
<infoitem key="RSS3_SITE_UTC_TIMEZONE" value="+09:00" />
<infoitem key="RSS3_CONFIG_CLOAKED" value="not include cloaked files" />
<infoitem key="RSS3_CONFIG_LASTMOD" value="ALL" />
<infoitem key="RSS3_CONFIG_MAXITEM" value="ALL" />
<infoitem key="RSS3_CONFIG_REFER_FLAG" value="YES" />
<infoitem key="RSS3_CONFIG_REFER_KEY" value="ref" />
<infoitem key="RSS3_CONFIG_REFER_VALUE" value="rss" />
</info>