Google ÜberProxy's Easter egg

It looks like the http://corp.google.com/ page has a fun Easter egg.

The page looks like a normal error page:

But if you hover over the "What's ÜberProxy?" link, it shows a hidden Easter egg:

To show/hide the "ÜP" image, it uses a simple CSS rule:

.easter-egg {
  display: none;
}

a.footer-link:hover + div.easter-egg {
  display: block;
}

The image is directly embedded in the HTML as a data URI:

In the old versions of the page, the image was always visible (https://web.archive.org/web/20130716091355/http://corp.google.com/):