This usually means that the user mixed up our lowercase/uppercase naming convention; see https://github.com/facebook/react/pull/3429#issuecomment-85166440.

We could add a message like

> Warning: The tag <commentBox> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter.

We'd check this only once per tag and only in development mode.

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', () => {
      if (ReactDOMFeatureFlags.useCreateElement) {
        spyOn(console, 'error');
        var defaultCreateElement = document.createElement.bind(document);
        try {
          document.createElement = element => {
            var container = defaultCreateElement(element);
            container.shadyRoot = {};
            return container;