Since teh 6.6.0 version and according to Atlassaian's documentation for the Jira renderers configuration all the fields are in plain text unless these listed below supporting wiki markup:

• Description
• Comment
• Environment
• Custom Fields

The fields above include HTML markups too.

A new function is supported to convert HTML to plain text:

• htmlText(String)

As Wiki markup in Jira is imported from Confluence, you have to include a CSS file in your HTML pages in order to display some contents properly:

${JIRA_BASE_URL}/download/resources/jira.webresources:global-static/wiki-renderer.css