> A specification is a document that describes a process or technology that is 
> sufficiently precise enough that people can use the document to implement 
> the process or technology in such a way that the desired output always flows 
> from a given set of inputs.

Actually, one of the main reasons for a specification is often to clearly
identify what is left as implementation choices; it is about identifying
what can be safely assumed, as much as exactly what happens.

CSS is a particularly strong case here, as, whilst many graphic designers
expect PDF like fidelity, CSS is actually only specified as hinting, and
that is a positive choice in its specification.