HTML can be used for notification message templates.
If the template body starts with <html> tag or <html lang="en"> tag (immediately after the top of template macros, if applicable and lang can be set to the appropriate language code), for notifications, the message will be sent as an HTML formatted message. The rest of the template should be created with HTML tags as needed. An example of an HTML formatting is:
<html><body>
<<#~Issues Tech>>
</body>
</html>