It's a little problematic to share an HTML file that you made or saved on your phone with other phone users directly, for example by sending via a messaging app: Android it's ok, but iOS won't open or save an HTML file sent in this way. Apparently there is a workaround to long press on the file to save it to your Files folder first.

This issue is relevant if your app's functionality includes the user changing the contents of the file and re-saving as a new file.