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.

var [attr, value] = expected;
      var re = '(?:^|\\s)' + attr + '=[\\\'"]';
      if (typeof value !== 'undefined') {
        re += quoteRegexp(value) + '[\\\'"]';
      }
      return new RegExp(re).test(actual);
    }
    function genMarkup(props) {
      return ReactDOMServer.renderToString(<div {...props} />);
    }
    it('should generate the correct markup with className', () => {
      expect(toHaveAttribute(genMarkup({className: 'a'}), ['class', 'a']));
      expect(toHaveAttribute(genMarkup({className: 'a b'}), ['class', 'a b']));
      expect(toHaveAttribute(genMarkup({className: ''}), ['class', '']));
    });
    it('should escape style names and values', () => {
      expect(
        toHaveAttribute(
          genMarkup({
            style: {'b&ckground': '<3'},
          }),
          ['style', 'b&amp;ckground:&lt;3;'],
        ),
      );
    });
  });
  describe('createContentMarkup', () => {
    function quoteRegexp(str) {