[CSS21] why keep S in the core syntax grammar rather than treat it like COMMENT?

In 4.1.1 Tokenization
  http://www.w3.org/TR/CSS2/syndata.html

I see:

  COMMENT tokens do not occur in the grammar (to keep it readable),
  but any number of these tokens may appear anywhere outside
  other tokens.

Why not do likewise for S? I'm pretty sure it doesn't change
the sequences of characters matched by the grammar.

In the implementation I'm writing, I moved S out of the
grammar and it seems to only help.

for the curious, see class CSSCore in
http://bitbucket.org/DanC/cssval/src/tip/src/main/scala/org/w3/cssval.scala

-- 
Dan Connolly, W3C http://www.w3.org/People/Connolly/
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