I know that you can query the content of a rich-text field as markdown via the API, maybe you can also specify the format “html”. I will have a look and report back. If it is not possible I can provide an API endpoint that offers the functionality „rich-text to html“ 
Update: Looks like this is supported, the format “html” is included in the Fibery API Docs:
curl -X GET https://YOUR_ACCOUNT.fibery.io/api/documents/b33a25d1-99ba-11e9-8c59-09d0cb6f3aeb?format=html \
-H 'Authorization: Token YOUR_TOKEN' \
The query parameter format=html should do the trick.