I downloaded 16 lessons to learn WebGL programming of this WebGL tutorial from GitHub, the first four lessons are fine and in the canvas HTML5 element I see what I should see, but in lessons 5 to 13, the canvas is entirely black and I don't see any colorful shape on it. In lesson 14, I only see in the center of the canvas the white text "Loading world...", the canvas in lesson 15 is again entirely black and in the last lesson number 16, I do see in the canvas a moving and turning white shaded box from left to right during few seconds and then it disappears and after that the canvas is entirely black again. According to his/her WebGL tutorial, the canvas in each lesson should show me nice colorful shaded and textured 3D shapes transforming in real time on the canvas, not everything black, but this is not happening for me. Why? I am using the latest version of Google Chrome internet browser. What can possibly cause this? In addition to the 16 lessons, there are also 3 examples, the first two examples are fine, i.e. in the canvas I see what I should see, but in the third example the canvas is entirely black again and I guess that this should not be happening.