alt Hacker NewsApple's native SwiftUI framework and Google's Jetpack Compose framework stumbled upon basically the same layout system, "Constraints down, sizes up":
1. Proposal: The parent component proposes a size/constraints to the child
2. Measurement: The child component picks its own size based on those constraints
3. Placement: The parent component then positions the child in its coordinate space
It's all done in a single pass. It scales great. It fits great with React-like reactive UI frameworks. (SwiftUI and Jetpack Compose are based on React.)
This is what CSS needs.
That pretty much is how CSS works! At the most basic level, Flow level is about widths down, heights up. But this basic model doesn't let you do a lot of things some people want to do, like distributing left-over space in a container equally among children (imagine a table). So then CSS added more stuff, like Flex-box, which also fundamentally works like this though adds a second pass.