Responsive Web Design - test suite not working

I am stuck trying to test my first Responsive Web Design project. When I click on Run Tests I just get ‘Load Tests!’ on the button below, and an error appears at the bottom of my web page, saying ‘#mocha div missing, add it to your document’. The only time I get tests running is if I haven’t yet made a single change to the initial pen, the one that only has the <script ...> and the comments. Then all tests fail because there is no content but at least they run.

What do I need to do to make this work?

Post your pen(s) here, it is impossible to say anything like this

It doesn’t matter what is the content of the pen. Literally any content at all gives this problem. Except the completely pristine initial content you get when you click on ’ You can build your project by forking this CodePen pen’. If I change that in any way the problem begins.

I got it to work. After each edit in the Pen I need to Save it and reload the page. Then the tests work.