Also, I noticed that it requires WebGL so without modern and supported by WebKit graphic card it won't work. Why is OpenGL necessary for a text editor? I don't understand.
Hardware acceleration of rendering is more energy-efficient and less resource intensive than CPU-based software rendering.
They will render text to a bitmap using HTML canvas and then use that as a texture for GPU.
GPU will be compositing those bitmaps, it won't actually be rendering text.