@fortMaximus Cracked it! CodePen adds the latest alpha of Bootstrap 4.....but when I delete that and use version 3, everything works.
So I now have a stable dev platform to work with. Yeah!
https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css <== Worked (copied from Quincy's settings)
https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-alpha.6/css/bootstrap.min.css <== Didn't work (from "Quick -add" of Bootstrap)
I can now delete everything and start from scratch.