Hi @nicolask-robert , the closest clues I can find are in this 2018 IBM Support page:
IBM Support: Document manager view action on HTML files presenting unreadable information
https://www.ibm.com/support/pages/document-manager-view-action-html-files-presenting-unreadable-information
At the bottom of the page, it says: "Allowing the HTML content to be viewed by Document Manager will allow an attacker to upload a maliciously formed HTML, allowing a large number of html/javascript/java and other various attacks. That is why it is working as designed, showing the html source."
Regarding an alternative "modern way" to render this, let me contact the team. 🙂