What is the burden of proving that a federal constitutional error is harmless?

Regarding errors of federal constitutional dimension, however, defendant argues that because the beneficiary of federal constitutional error has the burden of showing that the error was harmless beyond a reasonable doubt (Chapman v. California, supra, 386 U.S. at p. 24, 87 S.Ct. 824 ), we should adopt the approach that federal courts use when the government fails to argue that an error is harmless: the issue is generally considered forfeited, but the court may exercise its discretion to overlook the forfeiture, considering (1) the length and complexity of the record, (2) whether harmlessness is certain or debatable, and (3) the futility and costliness of further proceedings in the trial court. (United States v. Giovannetti (1991) 928 F.2d 225, 226227.)