Formal validity of an instrument?

Fundamental

A renvoi is excluded in matters of the formal validity of an instrument.

This means that when the question arises of the formal validity of a will, it matters little that the law specified pursuant to the Regulation declines the offer of jurisdiction made to it (Article 27[1]).

The same applies to the formal validity of a declaration accepting the succession or not (Article 28 b[2]).

The idea here is as follows

The legislator has decided to establish an alternative rule of conflict with a view to favouring the formal validity of the will. By accepting the renvoi, there is a risk of reducing the terms of the alternative and therefore failing to achieve the objective of the rule.