foo should not contain script tag
Some feed elements are allowed to contain HTML. However, some HTML tags, like script, are potentially dangerous and could cause unwanted side effects in browser-based news aggregators. In a perfect world, these dangerous tags would be stripped out on the client side, but it's not a perfect world, so you should make sure to strip them out yourself.
The validator will flag any element that contains any of these HTML tags:
commentembedlinklistingmetanoscriptobjectplaintextscriptxmpRemove the offending HTML tags. At a minimum, ensure that your content will still display as intended if this element is stripped by security conscious clients.