testing_html - fixing syntax plus editorial

- Fixed some markdown indentation for better rendering.
- Updated localhost URL to a link for convenience.
- Toned down mentions of Spice, since this page is linked from and
  provided in non-Spice-specific places. Hopefully, the transition is
  not as jarring.

As is my wont, also includes other editorial changes since I was here
and doing edits, anyway.

This is a cleaned-up and checked-in-publisher version of my previous pull request.  I had no idea markdown could be so complicated.