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.
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 Zendesk 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?