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.

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