element should not contain
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. For this reason, these potentially dangerous tags are often stripped out on the client side.
Examples of HTML tags that the validator will flag:
Remove 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.
Not clear? Disagree?
You might be able to find help in one of these fine resources.