I added a logo image to an existing category (making no other changes to it) and the category home page started showing visible HTML markup in its description:
Removing the logo image made the markup go away. It appears the presence of the image changes the rendering of the description such that it gets run through an HTML-escaper filter.
The HTML markup also appears in the description field in the category edit pop-up. We created the description using the normal editing UI, as Markdown.
(We are running Discourse 1.1.0.beta4.)