Some other links you might find useful
3d game shaders for beginners has many great explanations of many graphics techniques. It is OpenGL based but the explanations are illustrated well and so it should be possible to adapt them to WebGL.
Learn OpenGL: Modern OpenGL lessons
These may or may not be useful. Though the APIs are similar OpenGL is not WebGL. For one OpenGL is a C based library. Another issue is OpenGL has many more features than WebGL and the shader languages have many differences. Still, many of the ideas and techniques shown are just as useful in WebGL as they are in OpenGL.
Spector: An extension to show all your WebGL calls
Shader Editor: An extension that lets you view and edit shaders in live web pages.
WebGL Insight: An extension to let you see WebGL usage
webgl-helpers: Scripts to help with WebGL
twgl: A library to help make WebGL less verbose.
PlayCanvas A WebGL game engine with game editor
regl: A stateless functional WebGL library.
WebGL: The WebGL spec
OpenGL ES 2.0: The spec WebGL is based on.
GLSL ES 1.0: The shader language spec for WebGL
Shadertoy.com: Amazing fragment shaders produced under extreme constraints
Warning: The shaders on shadertoy.com are usually not the kind of shader that are used in production WebGL apps. Still there are many techniques to be learned from their examples.
glslsandbox.com: The original fragment shader playground.
vertexshaerart.com: The vertex shader version of glslsandbox.
If you know of other good references to add feel free to open an issue.