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.
onError={e => console.error('onError called')}
          />
        </video>,
        container,
      );
      var errorEvent = document.createEvent('Event');
      errorEvent.initEvent('error', false, false);
      container.getElementsByTagName('source')[0].dispatchEvent(errorEvent);
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('onError called');
    });
    it('should not duplicate uppercased selfclosing tags', () => {
      spyOn(console, 'error');
      class Container extends React.Component {
        render() {
          return React.createElement('BR', null);
        }
      }
      var returnedValue = ReactDOMServer.renderToString(<Container />);
      expect(returnedValue).not.toContain('</BR>');
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        '<BR /> is using uppercase HTML.',
      );
    });
    it('should warn on upper case HTML tags, not SVG nor custom tags', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(
        React.createElement('svg', null, React.createElement('PATH')),
      );