As far as I know, there is no way to add attributes to HTML elements in the rich text editor.
Do you want your users (the people who use the Forest Admin) to be able to add HTML attributes in the text they type? Or do you want to automatically add attributes to HTML (let’s say the main div) to match the actual style in your app? If so I would suggest wrapping the content into a
div with a class created in your app where you can add style to children (e.g access it via
.my-rich-content > div > b).
Let me know if you have some more details about your use case. It will help us to create an entry in our product board that fits your actual need.