Therefore it makes zero sense to not support every css property whose meaning you understand in the next three months (the lifecycle), and in the next version you can correct it if it deviates.
The problem is basically that IE and Opera hasn't been run of the market yet.
I may be biased as long-time users often are, but they're definitely not comparable to the plague that is IE.
In addition I don't recall them being open source.
If they had taken on IE back in the day, it would have been something else but I just can't see why I should care today.
Like blind users are just a few percent? Or people with motor-control disabilities are just a few percent?
250 million users, across both desktop and mobile.
Major websites' patchy support for Opera helps local players ;)