I wrote hundreds of Rect components and what I learned is that Componets should be able to be styled by developer who is using it.

I think instead, there would need to be some special way to make the distinction of what is a slot attribute and what is a slot prop to be consumed with let:. Maybe a new directive like <slot attr:class="abc"/>?

feel like there needs to be an easy way to style sub-components without their cooperation

There is a good amount of properties that should mostly be applied from a parent's point of view. We're talking stuff like grid-area in grid layouts, margin and flex in flex layouts. Even properties like position and and the top/right/left/bottom following it in some cases.