Ask HN: What is the best specification you ever read?

A few days ago there was a discussion of John Carmack's "Quake C", and how it got people started programming.

I looked it up, and the specification/reference for it was incredibly readable.

So I'm curious: what were the best specifications (of the above type) that you've ever read - that really got you started with a new technology, or got it into your head very quickly, or simply were good for any other reason? What stands out as a technology or language standard or specification?