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.

});
    it('should warn about class', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(
        React.createElement('div', {class: 'muffins'}),
      );
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('className');
    });
    it('should warn about class (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(
        React.createElement('div', {class: 'muffins'}),
      );
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('className');
    });
    it('should warn about props that are no longer supported', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(<div />);
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(<div onFocusIn={() => {}} />);
      expectDev(console.error.calls.count()).toBe(1);
      ReactTestUtils.renderIntoDocument(<div onFocusOut={() => {}} />);
      expectDev(console.error.calls.count()).toBe(2);
    });
    it('should warn about props that are no longer supported (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(<div />);
      expectDev(console.error.calls.count()).toBe(0);