Svelte forces you do to do all kinds of things in a very specific way (as does every other framework/library), those constraints generally make for a better experience.

Ideally: Only let a parent control those specific CSS properties, and never let a child use them on the root element.

margin, flex, position, left, right, top, bottom, width, height, align-self, justify-self among other is CSS properties that should never be modified by the child itself. The parent should always have control of those properties, which is the whole reason I'm asking for this.

By adding constraints, we have managed to create a simple API and a performant output.

Instead, my purpose is to illustrate why an alternative approach — based on Single File Components with real CSS — is so damn delightful.

Designing with constraints. Using inline styles, every value is a magic number. With utilities, you're choosing styles from a predefined design system, which makes it much easier to build visually consistent UIs.