Blending

To use blending, enable it during initialization.

gl.enable(gl.BLEND);

The Raster Operations stage's two inputs SRC and DST can be blended arbitrarily.

To accumulate all layers additively:

gl.blendFunc(gl.ONE, gl.ONE);

For transparency effects:

gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);