Procedural rendering routines often need pseudorandom numbers. For graphics rendering, we usually prefer correct-ish and fast code over properly "correct" code. My approach to random number generation is usually to intermittently seed a weak pseudo-random number generator, like a linear-feedback shift register, from a stronger source like Javascript's Math.random().
On the GPU, things are not so simple: we want a pseudorandom algorithm that is fast, local, and parallel. For compatibility, we should also have this RNG use an 8-bit RGBA texture for its internal state. Bitops are a bit tricky in WebGL shaders (though not impossible), so we'll use a linear congruential generator.
