The first version of csspp which with 100% coverage tests is now out. The .tar.gz is not yet available because SourceForge.net still has a few problems... but you can grab the source from the GIT repository.
The library (and thus command line tool) include support for a very large number of internal functions, variables, user defined functions (without support for the @return yet), nested rules, nested attributes, C/C++ like expressions, arrays, maps, color operations, etc.
The expression support the following types:

Integer (numbers without a period)

Decimal number

Percentage

Boolean (the identifiers "true" or "false", result of relational comparisons)

String

Color, as hash definitions (#121212) or an identifier naming a color (teal, chocolate, marron, etc.)

Unicode Range

URL

NULL
Most of the operators are supported:

 and +  unary operators to negate or compute the entity; these require spaces

**  the power operator works on numbers

*  multiply numbers, duplicate strings, compute unicode range intersections

/  divide numbers; some fields do not support this operator unless written between parenthesis or as the identifier div

%  compute modulos

+  add numbers, concatenate strings

  subtract numbers

<, <=, >, >=  relational operators work on numbers, booleans, and strings

=, !=  equality operators work on numbers, booleans, strings, and colors

&&,   logical and and or operators; work on booleans, numbers, etc.

?:  the C/C++ conditional operator

:=  variable assignment

()  array and map definitions
And expressions support many internal functions (still to be grown!):

algebra: abs, ceil, floor, log, max, min, random, round, sign, sqrt

boolean: not

color: alpha, blue, frgb, frgba, green, hsl, hsla, hue, lightness, red, rgb, rgba, saturation

conversion: decimal_number, identifier, integer, percentage, string, unit

string: str_length, unique_id

system: function_exists, global_variable_exists, if, inspect, type_of, variable_exists

trigonometry: acos, asin, atan, cos, sin, tan