Formal verification methodologies are more important than some cure-all, one-ring-to-own-them-all specification.

Formal specification by itself has been tried and failed as far back as Z notation. This is a perennial problem that cannot be solved with a panacea single specification standard because it is a process problem requiring a suite of tools to verify the products plus human habits, intention, and human effort to achieve, maintain, and improve upon excellence. While standards, simplicity, and uniformity help, there is still much too much irreducible complexity and minutiae that cannot be wished away by some sort glorified Cucumber testing or inventing the N+1 thing on top of (Isabelle, Idris, Coq, etc.) that wasn't invented here.

An uncomfortable truth is that 99.99% of current software development is nowhere close to rigorous. This isn't remotely a good thing™, especially for essential components.

See also: seL4.