Responsive background images with JavaScript

You could, of course, use a highly-compressed video instead, as some other sites do. Further, Disqus puts this animated GIF on their 404 page, which should, if everything goes well, be infrequently visited by users -- and bandwidth not sprayed needlessly across the web.

In any case, the technique is equally useful for still images and was originally designed for the JPEG background on my homepage. That background also makes use of the new responsive img "srcset", which makes it further optimized for mobile users than using one JPEG for all.