foo should not contain script tag

Some RSS 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:

script

meta

embed

object

Remove the offending HTML tags.