In CSS Values 4, 10.9. Type Checking:

Math functions themselves have types, according to their contained calculations:

• [...]
• The type of a sign() expression is «[ "number" → 1 ]».
• The type of a sin(), cos(), or tan() expression is «[ "number" → 1 ]».
• [...]
• The type of a pow(), sqrt(), log(), or exp() expression is «[ "number" → 1 ]».

[...]

A math function resolves to <number>, <length>, <angle>, <time>, <frequency>, <resolution>, <flex>, or <percentage> according to which of those productions its type matches.

I do not understand the type defined for these functions. I expect «[ ]» because from CSS Typed OM:

A type matches <number> if it has no non-zero entries and its percent hint is null.