Can a CPU implement RDRAND however it wants, or are there specifications that constrain it in some way? E.g. if a CPU were to put a microscopic lavalamp and ca

Can a CPU implement RDRAND however it wants, or are there specifications that constrain it in some way? E.g. if a CPU were to put a microscopic lavalamp and camera inside it and use a hash code of pictures taken from this lavalamp as the result of RDRAND: would that be compliant? I know microscopic lavalamp in CPU is not physically feasible, what I'm asking is if RDRAND is broken by design, or proper physics based RNG's could be used to implement if a CPU maker wanted to