It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
render() {
              return {x: 123};
            }
          }
          await render(<ObjectComponent />, 1);
        },
        ReactDOMFeatureFlags.useFiber
          ? 'Objects are not valid as a React child (found: object with keys ' +
              '{x}). If you meant to render a collection of children, use ' +
              'an array instead.'
          : 'A valid React element (or null) must be returned. ' +
              'You may have returned undefined, an array or some other invalid object.',
      );
      itThrowsWhenRendering(
        'top-level object',
        async render => {
          await render({x: 123});
        },
        ReactDOMFeatureFlags.useFiber
          ? 'Objects are not valid as a React child (found: object with keys ' +
              '{x}). If you meant to render a collection of children, use ' +
              'an array instead.'