Some feeds, like that of Diigo [https://www.diigo.com/rss/user/Agharbeia ] erroneously includes HTML markup in the <description> element of RSS.
In this specific case they use it to encode tags/keywords of bookmark items, where they should be using the <category> element for that.

The HTML markup causes havoc in the wikipage and editor.

It's better to sanitise content of <description>, and probably other elements, before emitting them to the page.