[css-variables] serializing cascading variable names

The CSS Variables spec has a one-line section noting [*]:

> Custom property names must be serialized exactly as provided by the author.

I don't think this is quite right.  They should be serialized as
identifiers are serialized which may or may not be "exactly as
provided by the author".  Escape sequences may be trimmed out for
example if not needed.

Cheers,

John Daggett

[*] http://dev.w3.org/csswg/css-variables/#serializing-custom-props