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 <div style={{margin: '0'}} />;
        }
      }
      ReactTestUtils.renderIntoDocument(<Component />);
      expectDev(console.error.calls.count()).toBe(0);
    });
    it('should warn nicely about NaN in style', () => {
      spyOn(console, 'error');
      var style = {fontSize: NaN};
      var div = document.createElement('div');
      ReactDOM.render(<span style={style} />, div);
      ReactDOM.render(<span style={style} />, div);
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toEqual(
        'Warning: `NaN` is an invalid value for the `fontSize` css style property.',
      );
    });
    it('should update styles if initially null', () => {
      var styles = null;
      var container = document.createElement('div');
      ReactDOM.render(<div style={styles} />, container);
      var stubStyle = container.firstChild.style;
      styles = {display: 'block'};
      ReactDOM.render(<div style={styles} />, container);
      expect(stubStyle.display).toEqual('block');