Margins and align-self aren’t side-effects. Their semantics are consistent and reproducible and depend only on the context a component is placed in, just as a property like width does.

They’re awkward because they have bigger knock-on effects on the overall layout of components within the parent.

In an OO mindset, I see this as being all about what interfaces your object exposes. For something like a button or slider control, it obviously exposes an action callback of some kind. But it also exposes a generic “child component” interface, which is the only thing a generic parent component cares about.

What are useful things and what are obnoxious things for a child component to do? Flexibly adapting itself to a range of sizes is useful. Demanding that it should be centered is obnoxious. In fact it would be hard to express at all in most OO widget toolkits. So I think this problem is very closely related to good OO design.