WebGL is a browser technology that enables hardware-accelerated 3D graphics using 2D APIs. It requires programmers to provide vertex and fragment shaders that specify 2D clipspace coordinates and colors, respectively, for WebGL to render real-time 3D graphics. While CSS3 makes building rich sites and apps easier with new features like 2D transforms, transitions, 3D transforms and webfonts, WebGL focuses specifically on displaying 3D graphics through 2D APIs and shader programming.