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('should throw when an attack vector is used', () => {
      var hackzor = React.createElement('div><img /><div');
      expect(() => ReactTestUtils.renderIntoDocument(hackzor)).toThrow();
    });
  });
  describe('nesting validation', () => {
    it('warns on invalid nesting', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(<div><tr /><tr /></div>);
      var addendum = ReactDOMFeatureFlags.useFiber
        ? '\n    in tr (at **)' + '\n    in div (at **)'
        : ' See div > tr.';
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: validateDOMNesting(...): <tr> cannot appear as a child of ' +
          '<div>.' +
          addendum,
      );
    });
    it('warns on invalid nesting at root', () => {
      spyOn(console, 'error');
      var p = document.createElement('p');
      ReactDOM.render(<span><p /></span>, p);
      var addendum = ReactDOMFeatureFlags.useFiber
        ? 
          '\n    in p (at **)' + '\n    in span (at **)'
        : ' See p > ... > p.';
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: validateDOMNesting(...): <p> cannot appear as a descendant ' +