API description
If you are interested only in the javascript API, I would suggest
Microsoft WebGL API (IE11 ?)
Mozilla WebGL API
As a starter I would suggest Microsoft one as it describes:
- extensions
- objects
- methods
- properties
- GLSL errors
It really focus on the API which makes it easier to explore IMO. You can find a pretty exhaustive list of methods. Note that Microsoft does not seem to implement all the features of WebGL if you compare it to Mozilla one.
On Mozilla you will find other very interesting resources (tutorials etc.). Of course another good place to get information is the kronos website. You will find the specification and reference card.
https://www.khronos.org/webgl/
For as why you cannot find a proper web page describing the Javascript API I would refer to Agost Biro answer
Tutorials
http://webglfundamentals.org/
http://learningwebgl.com/blog/
https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API/Tutorial
From those ones you will find even more resources.
