When a calc() function's simplified expression is a real number halfway between 2 adjacent numbers and the property accepts only an integer, then such simplified expression should be rounded _ towards positive infinity _.
if a math function that resolves to <number> is used somewhere that only accepts <integer>, the computed value and used value are rounded to the nearest integer, in the same manner as clamping, above. The rounding method must be the same as is used for animations of integer values.
CSS4 Values, section 9.1.4. Range Checking
(...) interpolation happens in the real number space as for <number>s, and the result is converted to an <integer> by rounding to the nearest integer, with values halfway between adjacent integers rounded towards positive infinity.
CSS4 Values, section 5.1.1. Combination of <integer>
z-index: calc(-3 / 2); /* -1.5 should be rounded up to -1 */
More info in this thread:
Right now, no browser passes the
z-index: calc(-3 / 2)