Hi @jokeyrhyme, it is a subtly different problem space than what the css-layout-api is trying to solve.
Most use-cases for ResizeObserver are to create/remove DOM based on you current size. A lot the use-cases for "element queries" are satisfied by this. For example when a menu-bar in an app is too small, display a drop-down-menu with the items instead.
The css-layout-api is explicitly trying to write css layouts in author code performantly, at this stage of the pipeline you don't have elements to refer to and manipulate.