Hi 4edu, hope you're doing well. I’m Ian, an Independent Advisor and Microsoft user like you. I’m happy to help answer your questions.
I'm sorry to hear about this issue.
Unfortunately I'm sad to say that Powerpoint doesn't have any feature to save pptx to html file. Microsoft doesn't have any tool that converts the file to html.
What I can suggest is to convert the file into video (mpeg-4) and embed this file to your html.
I would suggest to submit feedback to the development team in the link below so that they can add this in the future releases.
https://powerpoint.uservoice.com/forums/288949-...
This is a user-to-user support forum and I am a fellow user.
I hope this helps, but please let me know if you need anything else.