WebCore/platform/Length.h defines the following functions:
bool isZero() const
bool isPositive() const
bool isNegative() const
A calculated value in a Length is always a mixed percent/absolute, eg -webkit-calc(100% - 10px). Without a maxValue (ie the value that equates to 100%) passed in to these functions, we cannot determine the sign or the zero-ness of the result.