Dynamic content supports the use of HTML and Markdown however there are limitations when a dynamic content placeholder is used in the trigger message body, as well as agent signatures.
HTML in a dynamic content snippet will render in comments that are not used by a trigger or automation. It will not render if the dynamic content placeholder is inside a macro which is fired in a comment
Markdown and HTML from dynamic content are supported in the ticket comment body but don't render when the dynamic content placeholder is used in the trigger message body and then sent in a Support email notification. The same is true for agent signatures. Instead, use HTML without dynamic content in the trigger body to add formatting to emails, and use Markdown for agent signatures.
For more information, see the article: Where can I use Markdown and HTML in Zendesk Support?
not if it is used by a trigger or automation
does NOT render if the dynamic content placeholder is placed in a Macro and that macro is fired in a comment