Koi Dither Pond
Move your cursor over the pond and the koi dart away from it, then drift back once you settle.
Two shaders run as one. The first draws the fish, the second dithers whatever sits beneath it into two tones, and both are fused into a single draw call.
The two tones are the page's own accent and background, read from CSS, so switching theme repaints the pond without rebuilding the shader.