This usually means that the user mixed up our lowercase/uppercase naming convention; see https://github.com/facebook/react/pull/3429#issuecomment-85166440.

We could add a message like

> Warning: The tag <commentBox> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter.

We'd check this only once per tag and only in development mode.

ReactDOM.render(<div is="custom-div" />, container);
        expect(document.createElement).toHaveBeenCalledWith(
          'div',
          'custom-div',
        );
      } else {
        expect(
          ReactDOMServer.renderToString(<div is="custom-div" />),
        ).toContain('is="custom-div"');
      }
    });
    it('should work load and error events on <image> element in SVG', () => {
      spyOn(console, 'log');
      var container = document.createElement('div');
      ReactDOM.render(
        <svg>
          <image
            xlinkHref="http://example.org/image"
            onError={e => console.log('onError called')}
            onLoad={e => console.log('onLoad called')}
          />
        </svg>,
        container,
      );
      var loadEvent = document.createEvent('Event');
      var errorEvent = document.createEvent('Event');
      loadEvent.initEvent('load', false, false);
      errorEvent.initEvent('error', false, false);
      container.getElementsByTagName('image')[0].dispatchEvent(errorEvent);
      container.getElementsByTagName('image')[0].dispatchEvent(loadEvent);