An image can overflow

I’ve seen this used before to add a credit that only appears when you save the image. You append the credit to the bottom of the image, then crop the image with CSS using a technique like this to hide the credit. Then when somebody saves the image, they get the whole thing and the credit appears.

> Here is a demo where you can hover the image to show/hide the overflow:

Is something supposed to happen when I hover my mouse over the image? Because nothing happens. I'm using Firefox.