Views CSS class is changed when underscores and dashes are mixed

There is nothing in https://www.w3.org/TR/CSS21/syndata.html#characters as far as i can tell requiring that once you use a dash, you can't go back, or any other such bizarre rule about names for classes in HTML and CSS.

The double underscore is what triggers this and the fact that you get 2 variations is because of a BC layer.
Removal of the BC layer should be done in #2977950: Decide what to do with the BC css class from the Views UI → and should have been done already, but never got done.

Original issue : #2849954: Double underscores are not preserved in main views CSS classes defined in views UI → and CR https://www.drupal.org/node/2855055 →

So my suggestion would be to close this as a duplicate of the issue to get rid of the BC layer

Doing what I said in #4