If I had to guess, I'd say it's the third one. Given that Mozilla and Google have already publicly objected to this proposal, I don't expect them to implement

If I had to guess, I'd say it's the third one. Given that Mozilla and Google have already publicly objected to this proposal, I don't expect them to implement it. The W3C's word is not the law; no one is obligated to implement every specification they put forth.