The current text gadget in JIRA supports HTML to be displayed in the individual dashboard (and it can be individually customized). However, Text Gadget has a potential security risk. As this contains HTML which, in nature, does not have prevention from attacks.

Considering Text Gadget is the only text-display gadget that can be individually customized (compared to Introduction gadget and Announcement Banner), and that it is a very useful for customers not only in Cloud side as raised here JRA-29848 but also for JIRA Server, perhaps we could have a better approach for this. Maybe using the wiki-style renderer or even a plain text.

Workaround

There is a marketplace add-on called Rich Text Gadget for Jira by Atlassian Labs