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.

expect(stubStyle.fontFamily).toEqual('Arial');
      expect(stubStyle.lineHeight).toEqual('1.2');
      styles.fontFamily = 'Helvetica';
      ReactDOM.render(<div style={styles} />, container);
      expect(stubStyle.display).toEqual('none');
      expect(stubStyle.fontFamily).toEqual('Arial');
      expect(stubStyle.lineHeight).toEqual('1.2');
      styles.lineHeight = 0.5;
      ReactDOM.render(<div style={styles} />, container);
      expect(stubStyle.display).toEqual('none');
      expect(stubStyle.fontFamily).toEqual('Arial');
      expect(stubStyle.lineHeight).toEqual('1.2');
      ReactDOM.render(<div style={undefined} />, container);
      expect(stubStyle.display).toBe('');
      expect(stubStyle.fontFamily).toBe('');
      expect(stubStyle.lineHeight).toBe('');
    });
    it('should throw when mutating style objectsd', () => {
      var style = {border: '1px solid black'};
      class App extends React.Component {
        state = {style: style};
        render() {
          return <div style={this.state.style}>asd</div>;
        }
      }
      ReactTestUtils.renderIntoDocument(<App />);
      expectDev(() => style.position = 'absolute').toThrow();
    });
    it('should warn for unknown prop', () => {
      spyOn(console, 'error');