It seems problematic to allow arbitary non-lowercase attribute on HTML nodes. Because the server has no ability to keep them case sensitive (HTML isn't). So they will end up different on the client.

We should probably warn if the attribute is not a known one *and* is not lowercase.
);
    });
    it('removes a property when it becomes invalid', function() {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div whatever={0} />, container);
      ReactDOM.render(<div whatever={() => {}} />, container);
      var el = container.firstChild;
      expect(el.hasAttribute('whatever')).toBe(false);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid prop `whatever` on <div> tag.',
      );
    });
    it('warns on bad casing of known HTML attributes', function() {
      spyOn(console, 'error');
      var el = ReactTestUtils.renderIntoDocument(<div SiZe="30" />);
      expect(el.getAttribute('size')).toBe('30');
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid DOM property `SiZe`. Did you mean `size`?',
      );
    });
  });
  describe('Object stringification', function() {
    it('allows objects on known properties', function() {
      var el = ReactTestUtils.renderIntoDocument(<div acceptCharset={{}} />);
      expect(el.getAttribute('accept-charset')).toBe('[object Object]');
    });
    it('should pass objects as attributes if they define toString', () => {
      var obj = {
        toString() {