Browser support

Also list feature support, usage and how to detect support

Learning and reference

WebGL Fundamentals
WebGL from the ground up. No magic.

WebGL 2 Fundamentals
WebGL2 from the ground up. No magic.

Learning WebGL
Learning WebGL

Learning Three.js
Learning Three.js

Online courses (video)

Interactive 3D Graphics: Creating Virtual Worlds

Interactive Computer Graphics with WebGL (Angel)

Introduction to WebGL 3D with HTML5 and Babylon.js

Computer Graphics (UC San Diego)

Foundations of Computer Graphics (BerkeleyX)