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.
var el = ReactTestUtils.renderIntoDocument(<div whatever={3} />);
      expect(el.getAttribute('whatever')).toBe('3');
    });
    it('will not assign a function custom attributes', function() {
      spyOn(console, 'error');
      var el = ReactTestUtils.renderIntoDocument(<div whatever={() => {}} />);
      expect(el.hasAttribute('whatever')).toBe(false);
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Invalid prop `whatever` on <div> tag',
      );
    });
    it('will assign an object custom attributes', function() {
      var el = ReactTestUtils.renderIntoDocument(<div whatever={{}} />);
      expect(el.getAttribute('whatever')).toBe('[object Object]');
    });
    it('allows cased data attributes', function() {
      var el = ReactTestUtils.renderIntoDocument(<div data-fooBar="true" />);
      expect(el.getAttribute('data-foobar')).toBe('true');
    });
    it('allows cased custom attributes', function() {
      var el = ReactTestUtils.renderIntoDocument(<div fooBar="true" />);
      expect(el.getAttribute('foobar')).toBe('true');
    });
    it('warns on NaN attributes', function() {
      spyOn(console, 'error');
      var el = ReactTestUtils.renderIntoDocument(<div whatever={NaN} />);
      expect(el.getAttribute('whatever')).toBe('NaN');
      expectDev(console.error.calls.argsFor(0)[0]).toContain(
        'Warning: Received NaN for numeric attribute `whatever`. If this is ' +
          'expected, cast the value to a string.\n    in div',