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 warn about incorrect casing on event handlers (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(
        React.createElement('input', {type: 'text', onclick: '1'}),
      );
      ReactDOMServer.renderToString(
        React.createElement('input', {type: 'text', onKeydown: '1'}),
      );
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('onClick');
      expectDev(console.error.calls.argsFor(1)[0]).toContain('onKeyDown');
    });
    it('should warn about incorrect casing on properties', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(
        React.createElement('input', {type: 'text', tabindex: '1'}),
      );
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('tabIndex');
    });
    it('should warn about incorrect casing on event handlers', () => {
      spyOn(console, 'error');
      ReactTestUtils.renderIntoDocument(
        React.createElement('input', {type: 'text', onclick: '1'}),
      );
      ReactTestUtils.renderIntoDocument(
        React.createElement('input', {type: 'text', onKeydown: '1'}),
      );
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(console.error.calls.argsFor(0)[0]).toContain('onClick');