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.
expectDev(console.error.calls.count()).toBe(1);
        expectDev(console.error.calls.argsFor(0)[0]).toContain(
          'A component is using shady DOM. Using shady DOM with React can ' +
            'cause things to break subtly.',
        );
        mountComponent({is: 'custom-shady-div2'});
        expectDev(console.error.calls.count()).toBe(1);
      } finally {
        document.createElement = defaultCreateElement;
      }
    });
    it('should treat menuitem as a void element but still create the closing tag', () => {
      
      spyOn(console, 'error');
      var container = document.createElement('div');
      var returnedValue = ReactDOMServer.renderToString(
        <menu><menuitem /></menu>,
      );
      expect(returnedValue).toContain('</menuitem>');
      expect(function() {
        ReactDOM.render(<menu><menuitem>children</menuitem></menu>, container);
      }).toThrowError(
        'menuitem is a void element tag and must neither have `children` nor use ' +
          '`dangerouslySetInnerHTML`.',
      );
    });
    it('should validate against multiple children props', () => {
      expect(function() {
        mountComponent({children: '', dangerouslySetInnerHTML: ''});
      }).toThrowError(