background-size: cover

Try resizing this page, and watch what happens to the page's background image. This page has the following CSS specified for the body element (NB: distinct from the area containing this explanatory text):

  background-image: url(cowbell.jpg);
  background-attachment: fixed;
  background-repeat: no-repeat;
  background-size: cover;
  -moz-background-size: cover;

The background image is this image, displayed at original size, from the Wikimedia Commons; see COPYING for usage details: