Things have moved on since that guide was written, as you have found. No real need to test for "development" with this example, just use http://localhost/~ when testing with companion app or with a compiled app.
Nothing can be added to Assets after compiling. If your App is creating an HTML file, save it in the App's ASD. If you are downloading an HTML file, you can save that in the ASD too using an extension:
... or download to the /Download shared folder on your Smartphone.
It may actually still be pertinent for early version Android, and if used correctly still works. I have not had the time to check below Android 8.
Maintaining a website while providing technical support (free, we are volunteers) to people around the world is no easy task - there are just not enough hours in a day.
Taifun is one of the foremost experts in App Inventor - we are lucky to have his code snippets and extensions.
I think you are doing a great job.
You respond quickly and accurately to all the questions we ask you.
It was by no means my intention to criticize your work.
Back to my initial question, what I am trying to open in the webviewer is a .gpx file, not html. The html is already in assets.
I will now try with DownloadToASD.aix
I will let you know the results.
Thanks