> A generic SIMD API on a high-level language like JavaScript is as bad as it gets.

Dart VM SIMD API seems pretty nice to me: https://www.dartlang.org/articles/dart-vm/simd

  double computeAverage(Float32x4List list) {
    Float32x4 sum = new Float32x4.zero();
    for (int i = 0; i < list.length; i++) {
      sum += list[i];
    }
    // Perform horizontal operations once.
    double average = sum.x + sum.y + sum.z + sum.w;
    return average / (list.length*4);
  }

You may not be able to squeeze out the same perf, but seems worth having to me.

> JS engines to provide OpenCL-like GPGPU-targeted API

This is an interesting project, deep neural nets running on WebGPU: https://mil-tokyo.github.io/webdnn/