[SpecGL] C2: Identify what is required, Pcp 2

Principle: Explain which conformance requirements are mandatory, which
are suggested and which are optional

What does this mean?
Depending on the way you specify the conformance requirements, it may or
may not be clear if an implementation needs to implement all of them or
only part of them. Try and make sure one can easily distinguish the
level of requirements they have.

Why care?
If implementors don't have the same understanding of what's required,
interoperability is likely to suffer in the end.

Techniques
* use RFC Keyword (see Pcp 1)
* group the requirements of same level in subdivision of the technology
(see section @@@)
For instance, HTTP 1.1 defines two level of conformance, one where all
the MUST are respected, and one where all the MUST and the SHOULD are
implemented.
-- 
Dominique Hazaël-Massieux - http://www.w3.org/People/Dom/
W3C/ERCIM
mailto:dom@w3.org