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.
render() {
            return <polymer-component />;
          }
        }
        var node = document.createElement('div');
        ReactDOM.render(<ShadyComponent />, node);
        expectDev(console.error.calls.count()).toBe(1);
        expectDev(console.error.calls.argsFor(0)[0]).toContain(
          'ShadyComponent is using shady DOM. Using shady DOM with React can ' +
            'cause things to break subtly.',
        );
        mountComponent({is: 'custom-shady-div2'});
        expectDev(console.error.calls.count()).toBe(1);
      } finally {
        document.createElement = defaultCreateElement;
      }
    });
    it('should emit a warning once for an unnamed custom component using shady DOM', () => {
      spyOn(console, 'error');
      var defaultCreateElement = document.createElement.bind(document);
      try {
        document.createElement = element => {
          var container = defaultCreateElement(element);
          container.shadyRoot = {};
          return container;
        };
        mountComponent({is: 'custom-shady-div'});