A fragment is available for alpha blending only if it has passed the depth test. By default, depth testing is disabled and makes all fragments available for alpha blending.

Alpha blending is enabled using the following line of code:

gl.enable(gl.BLEND);

For each available fragment, the alpha blending operation reads the color from the framebuffer by the appropriate fragment coordinates and creates a new color based on a linear interpolation between the previously calculated color in the fragment shader and the color from the framebuffer.