How to show "blocked" icon when you hover over an element with the mouse

A few days ago I saw a 'block' icon when I put my mouse cursor over a link, indicating that I could not click. How can I do that? I do not know the correct term to search, maybe that's why I'm not finding it. Is it a plugin? An HTML5 feature?

Unfortunately I do not remember the site that I saw this on.

Thanks!

via Chebli Mohamed