What does the green label "Add." mean? on an extension in the chrome store?

Question

Google Chrome

spoiler

Answer 1

htmlbook
This property does not hide the element. If you want to hide the element itself, use display:none;

Answer 2

It's not at all clear where does overflow: hidden come from? It controls the content of the block, not the visibility of the block itself.
Now, if you put something inside, then it will make sense: https://jsfiddle.net/ukfm69xw/2/