To convert Word documents into HTML files, you may refer to this artcile "Save a document as a webpage".
But, based on my knowledge, the format of page on SharePoint is .aspx.
I find a thread, you may have a look, "Cant get html file to open in sharepoint".
(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
Besdies, if you have further questions related to SharePoint, I would suggest you post a new thread on Q&A forum with tag "office-sharepoint-online".
Thanks for your understanding.
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