It appears that I can upload ASPX files to Documents library in a regular SharePoint site and it will render. If I do the same to a Documents library in a SharePoint Site that was created as part of an Office 365 Group, the ASPX file will not render, but will be downloaded instead.
More details can be found here: https://sharepoint.stackexchange.com/questions/237928/how-do-i-allow-the-documents-library-in-an-off...
