Screen coordinates

I would like to know if there is any reference document that tells the viewable area on screen is initially X[-1, 1],Y[-1, 1],Z[-1, 1]. I can get this result by using vertices and using glReadPixels. But I need to refer it to an official document.

Alright, alright they are in range [1, -1] because of the homogenous coordinates stuff I suppose. But I still need a proof why they are initally between -1 and 1. Because of the multiplaction of some matrices? If so what are the initial values of those matrices. Can anyone show me (linear) algebraic representation of this?

This is because of the default OpenGL setup for matrices and viewport size.
The projection matrix is identity. Look at how an ortho matrix is built and it becomes clear.

For official documents download the GL spec from here
and read chapter 2.11 Coordinate Transformations and the tables in 6.9 for defaults of viewport, modelview and projection matrix.