Hi Greg,
You could take me demo as reference:
1. Create a HTML with code below: (I named it as RichTextRender.html)
Rich Text Render
The logic of my web resource is:
Query custom parameter from URL(the custom parameter here is a specific field of entity) -> Get value of the field -> Render the value(HTML source code) by setting them as HTML content of a div element
2. Pass a field as parameter to our web resource
e.g: I saved HTML code in description field of Contact entity, and I want to render them as rich text.

The URL of the web resource will be something like below when the web resource is loaded in form:(We can get the URL by browser inspector.)
https://crmxxx.crm.dynamics.com/{637268315600017751}/webresources/new_rich_text_render?data=description
The code in web resource will retrieve specific field value with value from data parameter.
3. Result:

(The web resource could only render HTML as rich text, I just copied HTML from this website https://html-online.com/editor/ for testing purpose, you can click QUICK TOUR to show demo HTML content if you can't see them by default.)
We can deploy the web resource to any form, and set any field as parameter.
Regards,
Clofly