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.
'Warning: Unknown event handler property `on-unknown`. It will be ignored.\n    in div (at **)',
      );
    });
    it('should warn for badly cased React attributes', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div CHILDREN="5" />, container);
      expect(container.firstChild.getAttribute('CHILDREN')).toBe('5');
      expectDev(console.error.calls.count(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Invalid DOM property `CHILDREN`. Did you mean `children`?\n    in div (at **)',
      );
    });
    it('should not warn for "0" as a unitless style value', () => {
      spyOn(console, 'error');
      class Component extends React.Component {
        render() {
          return <div style={{margin: '0'}} />;
        }
      }
      ReactTestUtils.renderIntoDocument(<Component />);
      expectDev(console.error.calls.count()).toBe(0);
    });
    it('should warn nicely about NaN in style', () => {
      spyOn(console, 'error');
      var style = {fontSize: NaN};
      var div = document.createElement('div');
      ReactDOM.render(<span style={style} />, div);
      ReactDOM.render(<span style={style} />, div);
      expectDev(console.error.calls.count()).toBe(1);