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(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Invalid props `foo`, `baz` on <div> tag. Either remove these ' +
          'props from the element, or pass a string or number value to keep ' +
          'them in the DOM. For details, see https://fb.me/react-unknown-prop' +
          '\n    in div (at **)',
      );
    });
    it('should warn for onDblClick prop', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div onDblClick={() => {}} />, container);
      expectDev(console.error.calls.count(0)).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Unknown event handler property `onDblClick`. Did you mean `onDoubleClick`?\n    in div (at **)',
      );
    });
    it('should warn for unknown string event handlers', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactDOM.render(<div onUnknown="alert(&quot;hack&quot;)" />, container);
      expect(container.firstChild.hasAttribute('onUnknown')).toBe(false);
      expect(container.firstChild.onUnknown).toBe(undefined);
      ReactDOM.render(<div onunknown="alert(&quot;hack&quot;)" />, container);
      expect(container.firstChild.hasAttribute('onunknown')).toBe(false);
      expect(container.firstChild.onunknown).toBe(undefined);
      ReactDOM.render(<div on-unknown="alert(&quot;hack&quot;)" />, container);
      expect(container.firstChild.hasAttribute('on-unknown')).toBe(false);
      expect(container.firstChild['on-unknown']).toBe(undefined);
      expectDev(console.error.calls.count(0)).toBe(3);