Wow, creating something interesting in under 256 bytes (for the 256b.htm compo) isn't easy... Here's one of the things I had to scrap: a random inkblot generator, way too big at 384 bytes... ;)

This one uses the XBM img src technique to generate a 2-bit image at run-time (example 1, 2). Could try rendering it with divs instead, but that'll probably cause quite the browser-freeze.