It is possible to perform some computations using OpenGL ES 3.0 / WebGL 2.0, but many types of operations (e.g. anything that involves random-access writes) are impossible, and many others (anything that normally requires shared memory) are very inefficient. Programming GPU through WebGL 2.0 is akin to programming desktop GPUs pre-CUDA: it is too intricate to take off.

Compute shader extension for WebGL 2.0 would be cool, but it would require to port a large part of OpenGL ES 3.1: OpenGL ES 3.0 / WebGL 2.0 doesn't include even random access buffers (SSBOs)