Blocks are server components

A block is invoked and its root cloned. A client reference cannot be invoked on the server, so a client block does not render at all — React throws Attempted to call X() from the server, which invokeBlock lets through untouched because the block's own failure is more informative than anything the renderer could substitute.

Supporting client blocks would need a second render path emitting createElement(component, …) without invoking, which makes data-nubbin-node part of every block's public prop contract and requires each client block to spread rest props onto its root. That is a contract no gate can check, failing silently as an unselectable region, in exchange for a case no consumer has asked for.

The alternative was a wrapper element around each block, which stamps reliably. It was rejected because it changes the consumer's layout, and Nubbin holds no opinion about styling.