Great question! I'm still experimenting with how responsive layouts should work, but so far: small/simple components like "Button" aren't responsive, while more complex components like "Hero" have responsiveness built-in.

At the css level, "top-level" components are absolute positioned, but nested components are layed out with flexbox.

There's no way to bring your own React component library yet, but once that's possible, you can always make things responsive in the library itself too.