WebGL is derived from OpenGL ES. WebGL 1 uses standards from OpenGL ES 2.0 and WebGL 2 uses standards from OpenGL ES 3.0.
[WebGL GLSL] stays very close to the OpenGL ES specification, with some concessions made for what developers expect out of memory-managed languages such as JavaScript.
So you will have to learn OpenGL ES GLSL. There is an official resource you can use to learn the theory and there is Shadertoy, where you can practice online and release/host your creations :)