What is the current state of the law on "beyond a reasonable doubt" standard?

"The beyond a reasonable doubt standard is a requirement of due process, but the Constitution neither prohibits trial courts from defining reasonable doubt nor requires them to do so as a matter of course. [Citation.] Indeed, so long as the court instructs the jury on the necessity that the defendant's guilt be proven beyond a reasonable doubt [citation], the Constitution does not require that any particular form of words be used in advising the jury of the government's burden of proof. [Citation.] Rather, 'taken as a whole, the instructions [must] correctly conve[y] the concept of reasonable doubt to the jury.' [Citation.]" (Victor v. Nebraska (1994) 511 U.S. 1 [----, 114 S.Ct. 1239, 1243, 127 L.Ed.2d 583].)