#1. Tentative definitions are historical baggage from Fortan Common blocks (
https://blogs.oracle.com/ali/entry/what_are_tentative_symbol...), which may have helped adoption of C by Fortran users. Although it remains in the C language specification, this feature can be ignored.
#2. Treating dereferencing a NULL pointer as undefined behavior means that the compiler is not required to generate additional instructions such as asserts or crashes to guard against potentially dangerous side effects. C compiler assumes that the programmer is in control of his/her code. In this example, it can be assumed that a careful C programmer has already guaranteed that the pointer will not be null when it is dereferenced. This C feature is an optimization to avoid generating redundant or unnecessary asserts or handling code.
3. C allows the programmer to handing pointers, allowing for such low-level optimizations that may not be possible in higher-level languages. A careful C programmer may have taken steps outside the function to handle the situation where yp==zp, or may otherwise be unconcerned about a particular case, for performance reasons.
4. A correct implementation of IEEE 754.
5. Since C is designed to efficiently compile to any computer architecture, it needs to be aware of the distinction between the arithmetic width of an instruction set vs the width of addresses. Ints are optimized to default to the natural arithmetic width of an instruction set (so that compilation doesn't produce unnecessary packing/unpacking instructions whenever they are accessed) but is guaranteed to be atleast 16 bits wide. However, since data structure can be as large as addressable memory, it is necessary for size_t to be the width of addresses.
6. Allowing size_t to be unsigned allows all bits of a size_t variable to be utilized for expressing size.
7. Undefined behavior is, again, an optimization feature, allowing each compiler to implement as it sees fit.
8. Comma operator is useful when first operand has desirable side effects, such as compactly representing parallel assignment or side effects in for loops.
9. C allows unsigned integers to wrap around 0 and UINT_MAX. This feature can be utilized as an optimization, for example as a free (no additional instruction) deliberate modulus operation. This is usually how unsigned integers behave in assembly.
10 & 11 & 12. Some ISA's, like MIPS, treat overflow of signed numbers as an exception. Others simply treat the result as a valid two's-compliment value. Since C is machine independent, C's official specification for overlow of signed numbers must be compatible for all ISA's. Simply treating the result as undefined does the trick, and means the compiler doesn't have to make guarantees or version for each ISA.