Anne van Kesteren for Mozilla says:
In other words, if your site isn’t HTTPS, you won’t get new web tech features. Holy jeepers. The reasoning is the web should be using HTTPS, so this is our way of beating you with a stick if you try to use fancy features without going HTTPS first.
It’ll be fascinating to watch the first major feature drop and if they stick to their word here. The web dev forums of the internet will overflow with WHY DOESN’T grid-gap WORK WITH MY FLEXBOX? (or some likely coming-soon feature) questions and the answer will be: talk to your server team. What if they drop container queries behind this? That would be a hilarious devastating tornado of developer fury.