WebGL video, where pixels with zero alpha are discarded?

I load a video to webGL:

  gl.bindTexture(gl.TEXTURE_2D, texture);
  gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, video);

The video has a part with alpha=0.0 but rgb != 0.0 (for next blending).
Inside the shader I see rgb = 0.0.
Where were the colors discarded and who did it?

video in format .webm with codec VP8.