Graphical layout is one of the most underestimated challenges in computer science. Programmers instinctively look at the problem and thing "oh, this is logical, I'll just whip up some basic principles and layout primitives and I'll have this solved in no time!"

No, you won't. Designing a layout framework is really, really hard. Designing one that is simultaneously expressive enough to handle all of the crazy corner cases that pop up on a daily basis AND simple enough to allow "normal people" to actually use and extend it is quite difficult. For example, the Android layout system is suitably expressive but totally mystifying to anyone who hasn't spent a year reading the source for View/ViewGroup/FrameLayout/ViewRootImpl.

The OP's suggestion that there is some singular, beautiful, composable layout system that we could drop in to replace CSS is, frankly, a pipe dream. Or rather, I would be far more convinced if he actually pointed to a spec or pre-existing system that we might base our new system off of [1].

I see only three real possibilities going forward, in order of increasing likelihood:

1. Someone does indeed come up with the One True Layout System (very unlikely).

2. We finally shove bytecode-level runtimes into a browser and let a thousand flowers bloom (even odds).

3. We keep incrementally improving CSS, adding support for various layout relationships that people need (probably).

[1] Elm's system looks neat, but it's not there yet.