Report authors can use the visual to write their own columns and measures using DAX, to create dynamic HTML content, or render existing content from their data model that contains HTML in their reports.
For information on getting started, worked examples, more detail on visual properties or version history, you can visit www.html-content.com to learn more.
(also available on the website)
For those who are more web development-savvy and want to attempt more advanced content, while the visual will have a good go at rendering the HTML content you supply, it only only passes your content into the DOM on your behalf, and what actually gets rendered can depend on a several factors.
We keep a living document of everything we currently know about on the website, but at a high level these include: