When I first tried to use both chroma and color matte nodes I noticed that the resulting matte was way too blocky. I noticed that only video files result in blocky matte. Stills do not get blocky. I guess Blender has a problem in extracting the right frame or enough info from the video clip to make a correct matte. The workaround for videos is quite simple: convert your video clip to still frames and add it to compositor as a still series. Is this a bug, should I report it as a bug? I never reported a bug and I do not know how to do it. If anyone else is willing to do it I will much appreciate it.