[css3-syntax] Normalize some tokens to lower-case

Hi,

Apparently at-keywords, function names and dimension units are 
normalized to lower-case in CSSOM serialization. Should that 
normalization happen as early as in the tokenizer?

Until tokens or "primitives" are exposed in some API this might only be 
an implementation concern and irrelevant to the spec. I’m not sure.

Cheers,
-- 
Simon Sapin