What is the test for defining a "vague" factor in a criminal case?

In the course of their analysis, the majority appear to suggest that if a factor is not vague, it necessarily passes muster under the United States Constitution, even if it "leave[s] the sentencer free to evaluate the evidence in accordance with his or her own subjective values." (Maj. opn., ante, at p. 398 of 15 Cal.Rptr.2d, p. 1158 of 842 P.2d.) Such a proposition would be too broad. The requirements imposed by the federal charter are substantive as well as formal. Thus, a factor, no matter [4 Cal.4th 598] how clearly defined, is constitutionally invalid if, for example, it authorizes or allows a juror to "attach[ ] the 'aggravating' label to" matters "that are constitutionally impermissible or totally irrelevant to the sentencing process, such as ... the race, religion, or political affiliation of the defendant" or "to conduct that actually should militate in favor of a lesser penalty, such as perhaps the defendant's mental illness." (Zant v. Stephens (1983) 462 U.S. 862, 885, 103 S.Ct. 2733, 2747, 77 L.Ed.2d 235.)