[css-syntax] Serialization round-trip

Hi,

The specification text that we had in the Serialization section was 
incorrect. [1] I just rewrote it to only specify normatively that 
serialization and parsing must "round-trip", and moved the details of 
how to achieve that into a non-normative note.

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

I would appreciate reviews of this section. In particular:

* Is there a case where doing as suggested does not actually round-trip?
* Is there a case that is likely to appear in real content where this 
suggests to insert a comment that is not actually necessary?


[1] Before today’s edits:

https://dvcs.w3.org/hg/csswg/raw-file/ecd7fe05fe45/css-syntax/Overview.html#serialization

Thanks,
-- 
Simon Sapin