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.

render() {
          return React.createElement('BR', null);
        }
      }
      var returnedValue = ReactDOMServer.renderToString(<Container />);
      expect(returnedValue).not.toContain('</BR>');
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<BR /> is using uppercase HTML.',
      );
    });
    it('should warn on upper case HTML tags, not SVG nor custom tags', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(
        React.createElement('svg', null, React.createElement('PATH')),
      );
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(React.createElement('CUSTOM-TAG'));
      expectDev(console.error.calls.count()).toBe(0);