face-attribute-relative-p

face-attribute-relative-p is a built-in function in `C source code'.

(face-attribute-relative-p ATTRIBUTE VALUE)

Check whether a face attribute value is relative.
Specifically, this function returns t if the attribute ATTRIBUTE
with the value VALUE is relative.

A relative value is one that doesn't entirely override whatever is
inherited from another face. For most possible attributes,
the only relative value that users see is `unspecified'.
However, for :height, floating point values are also relative.