To quote the person at my company who requested this:
“In the description and body content of each item, would you be able to remove any HTML markup, javascript, CSS, etc? In the past, we’ve crawled feeds including these, but those feeds tend to break when some random sequence of characters get introduced, which negatively affects search indexing.”
So, while the RSS feed is valid, the company’s internal search function seems to occasionally break the feed if it finds markup, javascript, or CSS. I know it’s a strange and very specific request.