This is about the overflow-x and overflow-y properties, which are explained here:
http://www.w3schools.com/cssref/css3_pr_overflow-y.asp
I thought I would understand the explanations, but when I used the "try it" editor, setting the overflow-y property in the example provided, to either 'scroll' or 'visible', produce both the same effect: A vertical scrollbar is shown. From the explanation, I expected to see a scrollbar in the example when overflow-y is set to scroll, but not when it is set to visible
I tried this on fairly new versions of Chrome, Firefox, Opera, and on IE11