I want the ability to flow content dynamically from one container to another using CSS

Flowing content from one container to another is an integral part of layout, yet it is missing on the web. The Regions specification has floated around for a while, but seems to have lost momentum. Meanwhile, the introduction of the grid layout module practically begs for this feature, whether you call it "Regions" or "dynamic content flow" or something else, to be available.

Imagine a layout where content flows dynamically from container to container depending on the available viewport. Imagine a layout in AR where content flows dynamically across containers placed in different locations. This feature is useful right now in the web-viewed-on-rectangular-screen modality and will become more important as the web migrates to new consumption modalities.

I have folders full of designs made impossible by the lack of dynamic content flowing, and I can think of a myriad of current and future use cases. This isn't just a "cool to have", this is an essential part of the evolution of web content layouts.