Unless if you think about it, layout is positioned in a 2D plane (with z-index, it's a 3D space, don't mention time), not a tree structure at all.
Except a tree structure is the only conceptual model that works for composing a user interface (out of simple, largely stateless components). See, for example, React and JSX.
default is "statically," if I may be so <strong>
Uh, yes, I guess? That by itself is not especially useful.
That's not what a layout language is for.
A layout language needs to be able to specify "A goes 10 px to the left of B".