as part of migration lot of html files have been moved to SharePoint 2013 to online. but the html files not working on the online.
You can Display the HTML page with Content Editor Web part (CEWP) by mentioning the location of the HTML file.
In SharePoint Online multiple links can be generate for an single item and here some links i have pasted below
Copy Links option : https://Tenant.sharepoint.com/:u:/r/Document/test.html?csf=1&e=RJulJe
So the above mentioned URL will not work to display the HTML page in CEWP, just provide the URL like https://tenant.sharepoint.com/sites/Documentlibrary/filename.html (this direct URL required to display the HTML page)