Rich Text Form Type 

Rich Text editor is based on TinyMCE.

HTML Purifier 

Rich Text editor uses HTML Purifier which helps to prevent XSS attacks. List of allowed HTML tags you can find in the app.yml file.

The following is an example of how to allow own HTML tags:

src/Acme/Bundle/DemoBundle/Resources/config/oro/app.yml 
 oro_form:
     html_purifier_modes:
         default:
             allowed_html_elements:
                 div:
                     attributes:
                         - data-url
                         - data-src
                         - data-value