In Box Generation, the spec says that none and contents cannot affect

whether a <legend> is the name of a <fieldset>.

But in Appendix B,

Per HTML, a <legend> with display: contents is not a rendered legend, so it does not have magical display behavior. (Thus, it reacts to display: contents normally.)

Not sure what exactly the first quote means with "name of a fieldset", my guess is that it refers to the rendered legend, but that contradicts the 2nd quote. If it means something else, please clarify.