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(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown event handler property `onUnknown`. It will be ignored.\n    in div (at **)',
      );
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(1)[0])).toBe(
        'Warning: Unknown event handler property `onunknown`. It will be ignored.\n    in div (at **)',
      );
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(2)[0])).toBe(
        'Warning: Unknown event handler property `on-unknown`. It will be ignored.\n    in div (at **)',
      );
    });
    it('should warn for unknown function event handlers', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div onUnknown={function() {}} />, container);
      expect(container.firstChild.hasAttribute('onUnknown')).toBe(false);
      expect(container.firstChild.onUnknown).toBe(undefined);
      ReactDOM.render(<div onunknown={function() {}} />, container);
      expect(container.firstChild.hasAttribute('onunknown')).toBe(false);
      expect(container.firstChild.onunknown).toBe(undefined);
      ReactDOM.render(<div on-unknown={function() {}} />, container);
      expect(container.firstChild.hasAttribute('on-unknown')).toBe(false);
      expect(container.firstChild['on-unknown']).toBe(undefined);
      expectDev(console.error.calls.count(0)).toBe(3);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown event handler property `onUnknown`. It will be ignored.\n    in div (at **)',
      );
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(1)[0])).toBe(
        'Warning: Unknown event handler property `onunknown`. It will be ignored.\n    in div (at **)',
      );
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(2)[0])).toBe(