How to allow display of unsafe content in Zendesk
By default, Zendesk is configured to block unsafe HTML tags and attributes, to ensure that your customers can safely browse your knowledge base. This may affect the way the browser renders the content, i.e., it may be displayed incorrectly or not displayed at all.
You can disable this default setting by allowing display of unsafe content:
1. In Guide, go to Settings>Guide Settings.
2. Find Security and check the Display unsafe content option.
3. Click Update.
Warning: this change allows potentially malicious code to be executed when users open an article in a browser.
More info on this question here.