Processing.js is not a "general purpose graphics library" like other JavaScript libraries, it is specifically meant to make Processing source code run on the web, and so if you run across bugs because ...

Use this browser-based application to create and render Processing programming language documents - 'sketches' - that have non-repeatable, randomized elements of form, shape, color and/or animation.

The Processing Javascript library has been adapted for use on the iPhone. iProcessing is an open programming framework to help people develop native iPhone applications using the Processing language.

Speaking of a programming language used for parallel computing using a GPU, Python is commonly used for research on machine learning, but you may want to use the GPU in a JavaScript web application.