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.
ReactDOM.render(
        <my-component>
          <my-second-component slot="first" />
          <button slot="second">Hello</button>
        </my-component>,
        container,
      );
      var lightDOM = container.firstChild.childNodes;
      expect(lightDOM[0].getAttribute('slot')).toBe('first');
      expect(lightDOM[1].getAttribute('slot')).toBe('second');
    });
    it('should skip reserved props on web components', () => {
      var container = document.createElement('div');
      ReactDOM.render(
        <my-component
          children={['foo']}
          suppressContentEditableWarning={true}
        />,
        container,
      );
      expect(container.firstChild.hasAttribute('children')).toBe(false);
      expect(
        container.firstChild.hasAttribute('suppressContentEditableWarning'),
      ).toBe(false);
      ReactDOM.render(
        <my-component
          children={['bar']}
          suppressContentEditableWarning={false}
        />,
        container,