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.
'Warning: Invalid DOM property `class`. Did you mean `className`?\n    in div (at **)',
      );
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(1)[0])).toBe(
        'Warning: Unknown event handler property `onclick`. Did you mean ' +
          '`onClick`?\n    in input (at **)',
      );
    });
    it('gives source code refs for unknown prop warning (ssr)', () => {
      spyOn(console, 'error');
      ReactDOMServer.renderToString(<div class="paladin" />);
      ReactDOMServer.renderToString(<input type="text" onclick="1" />);
      expectDev(console.error.calls.count()).toBe(2);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Invalid DOM property `class`. Did you mean `className`?\n    in div (at **)',
      );
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(1)[0])).toBe(
        'Warning: Unknown event handler property `onclick`. Did you mean ' +
          '`onClick`?\n    in input (at **)',
      );
    });
    it('gives source code refs for unknown prop warning for update render', () => {
      spyOn(console, 'error');
      var container = document.createElement('div');
      ReactTestUtils.renderIntoDocument(<div className="paladin" />, container);
      expectDev(console.error.calls.count()).toBe(0);
      ReactTestUtils.renderIntoDocument(<div class="paladin" />, container);
      expectDev(console.error.calls.count()).toBe(1);
      expectDev(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
        'Warning: Invalid DOM property `class`. Did you mean `className`?\n    in div (at **)',
      );