No really, they specified they are in twos complement, but the problem is the few types that applies to are optional:
220.127.116.11 Exact-width integer types
1 The typedef name intN_t designates a signed integer type with width N, no padding
bits, and a two’s complement representation. Thus, int8_t denotes a signed integer
type with a width of exactly 8 bits.
2 The typedef name uintN_t designates an unsigned integer type with width N. Thus,
uint24_t denotes an unsigned integer type with a width of exactly 24 bits.
3 These types are optional. However, if an implementation provides integer types with
widths of 8, 16, 32, or 64 bits, it shall define the corresponding typedef names.
It would be nice if they just specified everything had to be twos complement and maybe even little endian, but they are supporting ones complement, sign-magnitude and big endian and keep a huge number of things undefined. Hopefully someday computers running ones complement, sign-magnitude or big endian need not be considered anymore.