Check out these fun errors pages on Github. The server code 404 means that the page does not exist, and the code 500 means that the server had an error. See what happens when you move your mouse over the image after visiting the links! The technique used to move the images in such a way is called Parallaxing, where images that are intended to appear further from the viewer move less than the images that are intended to appear closer. A fun exercise would be to see if you can pick apart the source and replace the images with your own.