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.

ReactTestUtils.renderIntoDocument(React.createElement('IMG'));
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<IMG /> is using uppercase HTML.',
      );
    });
    it('should warn against children for void elements', () => {
      var container = document.createElement('div');
      expect(function() {
        ReactDOM.render(<input>children</input>, container);
      }).toThrowError(
        'input is a void element tag and must neither have `children` nor ' +
          'use `dangerouslySetInnerHTML`.',
      );
    });
    it('should warn against dangerouslySetInnerHTML for void elements', () => {
      var container = document.createElement('div');
      expect(function() {
        ReactDOM.render(
          <input dangerouslySetInnerHTML={{__html: 'content'}} />,
          container,
        );
      }).toThrowError(
        'input is a void element tag and must neither have `children` nor use ' +
          '`dangerouslySetInnerHTML`.',
      );
    });
    it('should include owner rather than parent in warnings', () => {
      var container = document.createElement('div');
      function Parent(props) {