Symptoms
Product descriptions and topic pages have HTML in them.
Cause
This is most likely because you pasted the code directly into the description field on the admin site, rather than the HTML editor.
Solution
Open the HTML editor from the menu bar above the description field (the button looks like <>) and paste the code into that window. If you do not see the menu bar above the description field, make sure that TurnOffHtmlEditorInAdminSite is set to false.