[css-syntax] Drafted a new "CSS Stylesheet" section, please comment

Hi,

The output of the parser described in css-syntax is made of generic 
qualified rules, at-rules, and declarations where rule preludes, block 
contents and declaration values can be almost anything.

As requested during the Tokyo F2F, I started a new section on how to 
interpret this output into a CSS stylesheet as represented in CSSOM, 
expected by Cascade, etc.

http://dev.w3.org/csswg/css-syntax/#css-stylesheets

Comments?

-- 
Simon Sapin