Yes, yes we did contribute the whole position-area stuff to the spec. But that shouldn’t cause differences between implementations.
I haven’t seen anyone write about Safari’s implementation yet… but also, it’s still in beta, still a work in progress. Any “evaluation of differences” will be quickly out of date.
Keep an eye on Safari Technology Preview. Keep updating to the latest beta throughout the summer. There are months to go before anyone should worry.
And yes, we should write an article!